ROHSTOFFE UND ANWENDUNGEN
RAW MATERIALS AND APPLICATIONS
Electrical resistance Electro-chemical corrosion Carbon black Specific surface area Structure
Fillers in elastomers exhibit a tendency for
flocculation respectively networking which has
a significant effect on the properties of a filled
rubber compound. Networking of fillers depends on their attractive potential and the
distance between their aggregates. Since, with
regard to surface energy [4], the attractive
potential is almost the same for all furnace
blacks in hydrocarbon elastomers, the distance
between aggregates becomes the controlling
factor for filler networking.
In this study it was pointed out that filler
networking has an influence not only on
conductivity but also on the dynamic stiffness
of a filled rubber compound. To find the right
balance of electrical and viscoelastic properties, which is required today for rubber sealings which separate metals of different electrochemical potentials to avoid electrochemical corrosion, first of all an analysis for the
percolation threshold for various blacks in a
practical EPDM formulation was done. The
second investigation contained an experimental design for special soft blacks in a very
basic formulation.
Einfluss der Struktur und spezifischen Oberfläche von Soft Carbon
Blacks auf den elektrischen Widerstand von gefüllten Gummimischungen
Elektrischer Widerstand Eletrochemische
Korrosion Ruß spezifische Oberfläche Struktur
Füllstoffe in Gummimischungen zeigen die
Tendenz zur Flokkulation bzw. zur Bildung
eines Füllstoffnetzwerkes, was einen beträchtlichen Einfluss auf die Eigenschaften
einer gefüllten Kautschukmischung hat. Die
Bildung eines Füllstoffnetzwerkes hängt
von den Anziehungskräften der jeweiligen
Füllstoffe sowie vom Abstand zwischen
einzelnen Füllstoffpartikeln ab. Da die
Wechselwirkungskräfte [4] für alle Furnace
Carbon Blacks in Elastomeren fast identisch
sind, stellt der Abstand zwischen den Füllstoffpartikeln den entscheidenden Faktor
für die Bildung eines Füllstoffnetzwerkes
dar.
In dieser Studie wurde gezeigt, dass die
Füllstoffnetzwerkbildung nicht nur einen
Einfluss auf die elektrische Leitfähigkeit
sondern auch auf die dynamische Steifigkeit einer gefüllten Kautschukmischung
hat. Heutzutage ist ein ausgewogenes
Verhältnis zwischen elektrischen und viskoelastischen Eigenschaften bei Gummidichtungen sehr wichtig, um elektro-chemische Korrosion an Metalle, die unterschiedliche elektrochemische Potenziale
aufweisen, zu vermeiden. Daher wurde als
erstes eine Analyse der Perkolationsschwelle von verschiedenen Carbon Blacks
in einer praxisnahen EPDM Rezeptur
durchgeführt. Im zweiten Schritt wurde zur
Bewertung spezieller Soft Carbon Blacks in
einer Basisrezeptur ein statistischer Versuchsplan angewandt.
Influence of Structure and
Specific Surface Area of Soft
Carbon Blacks on the Electrical
Resistance of Filled Rubber
Compounds
As lightweight design becomes more and
more important in the automotive industry, an increasing amount of aluminium
and magnesium parts is used in combination with steel for body constructions. Because of the variable electrochemical potential of these metals corrosion problems
can take place especially where parts of
the different materials make contact by
electrically conducting rubber sealings.
To overcome this problem of electrochemical corrosion, the usage of special soft
blacks is necessary in order to increase
the electrical resistance of the applied sealing rubber compounds. As a consequence
the influence of structure and specific surface area of carbon blacks on the electrical
behavior of filled rubber compounds is of
great interest and is investigated in detail.
Furthermore accompanying dynamic
stress-softening measurements (Payne effect) are carried out. The findings are related with electrical resistance behavior
to obtain more basic information about
the influence of the nature of carbon
blacks on this subject.
Electrical conductivity plays an important
role in many rubber articles including antistatic applications. Pure elastomers commonly act as electrical insulators. Conductivity to rubber compounds is imparted by
addition of a finely divided or colloidal filler
of high intrinsic conductivity, such as carbon black. At low loadings of carbon
blacks, the conductivity of the composite
is essentially that of a dielectric medium.
As loading is increased, a percolation
threshold or critical loading is reached
where the conductivity starts to increase
rapidly. After overriding the percolation
zone the con-ductivity respectively the resistivity of all blacks asymptotically reaches
an endpoint [1].
In most practical cases good conductivity
of rubber compounds is required, which
is possible by using a high amount of carbon black. But there are also occasional si-
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003
tuations where low conductivity is required, even in the presence of high carbon black loadings to ensure good physical
properties as well as excellent processing.
This is the case if light metals (aluminium,
magnesium) are in contact via rubber sealings with more precious metals. In the presence of an electrolyte the light metal will
be corroded preferentially. Consequently
such rubber sealings have to have a sufficient high electrical resistivity. In this investigation we are chiefly interested in the
electrical conductivity characteristics of
filled rubbers in dependence of carbon
blacks especially with very low surface
areas.
Electrical Conductivity
The conductivity imparted to a rubber
compound by a carbon black depends
mostly on the following parameters:
carbon black loading
primary particle size
carbon black structure
porosity
surface oxide groups
polymer, its chemical nature, molecular
weight and viscosity
mixing and finishing process [2]
Let us concentrate only on the influence of
primary particle size and structure of carbon black which are the most influencing
parameters.
Several authors [1, 3] have reported that
the primary particle size is the major carbon black parameter influencing conductivity. In order to assure electrical conductivity of rubber compounds through-going
paths for the current, built up from chains
of carbon black particles or aggregates, are
necessary, but not a direct contact of the
W. Niedermeier, J. Fröhlich, Hürth
519
Fig. 1. Typical volume resistivity curves as a function of filler loading
with various carbon blacks
aggregates. Electrical conductivity is ruled
by the gap widths between adjacent particles, aggregates or agglomerates. This
phenomenon can be explained by the concept of electron tunneling which is a quantum mechanical process. According to this
mechanism electrons may pass through
thin insulating polymer films separating
the carbon black particles. It is well-known
that the tunneling current is an exponential function of the gap width bet-ween
two particles. Thus, not the length of
the particle chains, but the average width
of the gaps between the particles determines the electrical conductivity of carbon
black loaded vulcanizates. Wang, Wolff
and Tan [4] had shown, that the main filler
parameter determining the distance between aggregates besides loading is the
specific surface area. Consequently, the
smaller the primary particle size respectively the aggregate size are at a fixed
structure and filler loading, the smaller
the gaps. The exponential dependence
of the current on the gap width then explains the strong impact of the interparticle
or interaggregate distances. Already small
changes in the gap width will strongly influence the conductivity.
The particle aggregation respectively the
carbon black structure has been defined
quantitatively by Medalia [5] as the average number of particles per aggregate.
Hence, the higher the structure is, the
more branched or porous are the aggregates. Janzen’s theory [6] predicts that
high-structure blacks should have a low
percolation threshold, and at a given loading, a high-structure black could be expected to have a higher conductivity
than a low-structure black. Indeed some
conductive blacks, such as Printex XE-2
520
Fig. 2. Volume resistivity as a function of filler loading for special low
surface area carbon blacks in EPDM
or acetylene black, have a high surface
as well as a high structure. Nevertheless
Medalia [7] showed that standard rubber
blacks at a fixed loading in various elastomers do not give the expected effect regarding structure. A reasonable explanation is that the blacks with the high structure are better dispersed under the same
conditions as low structure blacks. On
the other hand Medalia [8] and Probst
[9] have used the formula of Janzen with
noticeable success.
Fig. 1 represents typical electrical resistivity
data of rubber compounds containing various carbon black grades as a function of
the degree of loading. It seems that all resistivities tend to reach a similar asymptotic
limit, which is obtained at a much smaller
degree of loading for high surface area
blacks. The influence of the specific surface area can be recognized very easily
comparing the blacks N 220 and N 375,
which differ only in surface area. The percolation threshold respectively the critical
loading, which is located at the strongest
decrease of the resistance curve, arises at
lower loadings for N 220, the black with
the higher surface area. However the influence of the structure cannot easily be estimated by these measurements.
cured rubber sample of 82 mm in diameter
and a thickness of 2 mm was coated with
silver to have virtually no contact resistance.
Results
Fig. 2 shows that blacks which differ in
specific surface area as well as in structure
can be brought into the upper limiting conductive zone by simply increasing their
loading. Therefore at extremely high load-
Tab. 1. EPDM formulation
Stage I
[phr]
BUNA EP G 5455
Carbon black
ZnO
Stearic acid
LIPOXOL 4000
150
50 – 150
5
2
5
Stage II
[phr]
Vulkacit Mercapto C
Vulkacit Thiuram C TMTD
Rhenocure TP/S
Sulfur
Tab. 2. EDPM mixing procedure
Stage I
0–10
Experimental
The following investigations were carried
out in EPDM based on the formulation according to Tab. 1 and 2. The carbon blacks
used in this study are shown in Tab. 3. As
can be seen in this table all blacks are characterized by low specific surface areas and
relatively high structure levels. The measurements of the electrical resistivity
were carried out regarding DIN 53482. A
1
0.5
2
1.5
10
1–40
40
Polymer
Carbon black, ZnO
Stearic acid, Lipoxol 4000
Sweep
Mix
Dump at 100 – 160 8C
Stage II
0–20
20
Batch stage I
Vulkacit Mercapto C
Vulkacit Thiuram C
TMTD
Rhenocure TP/S
Sulfur
Dump at 100 – 130 8C
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003
Tab. 3. Analytical data of the carbon blacks A – E
Carbon Black
CTAB
DBP
CDBP
Modus
Mean aggregate size
D D50
2
[m /g]
[ml/100 g]
[ml/100]
[nm]
[nm]
[nm]
A
B
C
D
E
31
130
84
222
295
250
23
90
71
280
362
343
22
102
76
250
343
295
24
114
78
267
339
296
40
121
98
197
240
236
ings, the width of the gaps between the
aggregates seems to become very small
and similar for all carbon black grades.
As a consequence the electrical resistance
of the gaps is negligible and the conductivity of the compound is ruled by the intrinsic
conductivity of the carbon black aggregates. In practice of course an upper limit
is set to the loading by the viscosity during
compounding and by the hardness and
other physical properties of the vulcanizate.
Despite the fact that the differences in surface area are not so drastic the filler loadings to reach the critical point vary from
65 phr to 90 phr. A comparison of the
blacks, which differ only in structure shows
that a trend can be recognized. The higher
the DBP level of a black is, the more the
percolation threshold will be shifted toward a lower degree of loading (comparison of blacks B, C and D). To provide a
deeper insight into the phenomenon of
this critical filler volume the strain dependence of the complex modulus G* was
analyzed with the Rubber-Process-Analyzer (RPA) [10, 11]. The strain sweep measurements for the filled vulcanizates are
carried out at a frequency of 1.6 Hz and
a single strain amplitude in the range of
0.28 – 42 % (SSA).
It is well-known that after adding a filler to
an elastomer the low strain modulus G0
rises more than the high strain modulus
G1, resulting in a non-linear visco-elastic
behavior, known as Payne effect G0 –
G1. Therefore it can be expected that, if
the critical filler loading is reached to build
up a filler network throughout the whole
specimen, the Payne effect (G*(0.28 %) –
G*(42 %)) should be increased to a much
higher extent than at lower loadings. This
may be explained by the formation of a
“mechanical active” filler network, which
is stable at least against small dynamic deformations. At this point the question
arises: will these critical concentrations
for the formation of an electrical and “mechanical active” percolation threshold be
obtained at the same filler loading?
Fig. 3 shows the Payne effect for different
filler loadings. It can be seen very clearly
that the black A with the highest surface
area shows a considerable increase of
the Payne effect between 70 phr and
80 phr of filler loading. Comparing the
blacks B, C and D with similar surface areas
but different structure levels the increase of
the Payne effect for carbon black C with
the lowest DBP level can be only detected
at filler loadings of 90 phr to 100 phr.
At least a few maybe a single throughgoing carbon black path of sufficiently
low resistivity must exist to guarantee conductivity through the whole specimen.
Adding carbon black to an elastomer,
one by one, aggregates will at first be
separated, then separated agglomerates
will be formed (sub-networks), and finally
a through-going path will arise. At this
point conductivity is possible – the electrical percolation threshold is reached, but it
is ruled by a few, maybe only one of these
through-going paths. By further addition
of filler more agglomerates come into contact and a lot of such through-going paths
are formed, leading to a further improved
conductivity. This point is reached when
the most possible contacts are formed
and a so-called continuous filler network
is built up – the mechanical active percolation threshold is then reached. Comparing
now Fig. 2 and Fig. 3 it can be detected
that indeed, as described above, the percolation threshold levels are different. The
electrical percolation for black A is given
at about 65 phr (strongest drop in resistivity) whereas the “mechanical percolation”
is given just at about 75 phr (the first
strong rise in the Payne effect curve). It
should be noted, that reaching the “mechanical percolation” neither leads to a
modulus increase of some decades nor
to a final plateau of G*, like it is known
for conductivity. The name “mechanical
percolation threshold” is a creation and
only based on the fact, that a significant
increase in DG* at a certain filler level
can be observed. A similar behavior can
be seen for black D: electrical percolation
at about 85 phr and 95 phr for the “mechanical percolation”. For the remaining
two blacks the situation is not as clear as
described before. But it can be stated
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003
Fig. 3. Payne effect D G* ((G*0.28 %) –
G*(42 %)) vs. filler loadings for various carbon
blacks in EPDM
here that for the conductivity of a rubber
compound only a small number of
through-going paths of joined carbon
black aggregates seems to be necessary,
whereas for a mechanical active filler network, a continuous three-dimensional agglomeration through the whole specimen
has to be formed.
For a more detailed analysis of the influence of the specific surface area and the
structure on the electrical percolation
threshold resp. critical filler content a
more basic formulation was used
(Tab. 4). To separate the effects of these
main parameters an almost orthogonal experimental design was chosen. In order to
obtain an almost orthogonal design with
central point regarding the factors the investigated carbon blacks were selected as
displayed in Fig. 4. The analytical data of
the blacks used can be seen in Tab. 5.
The resistivity for different filler loadings
can be seen in Fig. 5. For the blacks 4, 5
and 6 which do not vary in surface area
the curves for the resistivity are very similar
and also the percolation threshold seems
to be the same. Comparing the blacks 1
and 2, which differ mostly in structure, it
can be recognized that the percolation
Tab. 4. Basic ESBR formulation
Stage I
[phr]
ESBR 1500
Carbon black
ZnO
Stearic acid
Wax
100
variable
3
2
1
Stage II
[phr]
CBS
Sulfur
1.5
1.5
521
Tab. 5. Analytical data of the blacks 1 – 6 used in the experimental design
Carbon
Black
CTAB
DBP
CDBP
[m2/g]
[ml/100 g]
[ml/100]
1
2
3
4
5
6
20
141
76
19
79
60
38
91
72
64
68
63
63
93
79
62
135
92
threshold for the black with the low structure is shifted towards higher filler loadings. The following can be stated: the lower the structure is, the higher the resistivity.
Very interesting is also the fact that, after
overriding the percolation threshold, the
resistivity studied at the same filler loading
is always higher for the black with the lower structure level.
Looking again more precisely on Fig. 2 the
same behavior can be detected. At practical filler loadings for this formulation
(110 phr to 140 phr) the compounds containing the blacks with lower structure
levels give higher resistivity according to
the order of the structure.
For the further evaluation of the influence
of the structure and the surface area on the
percolation threshold a double linear regression analysis with interaction was applied on the basic E-SBR formulation.
Firstly, the percolation threshold was estimated determining the logarithmic midpoint of the insulating and the conducting
plateaus from Fig. 5. To check, if this model
gives reliable data and if it can be used for
a solid prediction, let us look at Fig. 6.
Here, the observed vs. the predicted
threshold levels according to the model
are plotted. As can be seen, the chosen
model is a good one (R2 > 99 %).
Fig. 4. Array of the factors CTAB and CDBP for the experimental design
Fig. 5. Volume resistivity vs. filler loading for the blacks used in the
experimental design
Fig. 6. Predicted versus observed percolation threshold level
with the correlation
coefficient of
R2 ¼ 99 %
522
Fig. 7 shows the response surface area of
the percolation threshold (critical filler content).
The following can be observed: At high
specific surface areas the influence of
the crushed DBP absorption on the percolation threshold can be neglected. On the
other hand, the lower the surface area becomes, the more the influence of the
crushed DBP absorption rises and can
therefore no longer be neglected. Nevertheless Fig. 7 demonstrates, that the surface area is the dominating parameter.
The main effect of surface area is a decrease of the percolation threshold of
about 8 phr by an increase of the surface
area of 10 m2/g, whereas a rise in crushed
DBP of 10 ml/100g has only an effect of
2 phr (see Fig. 8 and 9).
Taking DBP instead of CDBP as factor the
effect of structure on the critical filler content would be still lower because of the
wider spreading of the DBP. Fig. 9 again
depicts that the influence of the crushed
Fig. 7. Results of the double linear regression
analysis regarding the response percolation
threshold (PT)
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003
Fig. 8. Influence of the specific surface area on the percolation threshold
Fig. 9. Interaction regarding surface area (CTAB: high and low) and
crushed DBP absorption on the percolation threshold
Fig. 10. Modulus 200 % as a function of specific surface area and crushed DBP
Fig. 11. The ratio M200/M50 as a function of
specific surface area and crushed DBP
DBP absorption becomes only significant
for the low surface area blacks of this
study.
As mentioned before the balance between
low electrical conductivity and good physical properties of certain compounds is of
great importance. Therefore also a detailed
analysis of the physical properties for a
fixed degree of filler loading of 60 phr
was carried out. The reinforcement potential expressed by both modulus 200 % and
the ratio M200 % and M50 % is of high
interest for blacks with a very low specific
surface area and a high structure level used
in extrusion articles. The dependence of
these parameters on surface area and
structure can be seen in Fig. 10 and 11.
It can be recognized very clearly that the
crushed DBP absorption has a much greater influence on the modulus 200 % than
the specific surface area expressed as
CTAB adsorption. Remarkable is the fact
that the higher the crushed DBP absorption is, the larger the influence of the sur-
face area. The plot of the ratio of M200/
M50 displays again the superior impact
of the crushed DBP in comparison to the
surface area. But the influence of the
crushed DBP for low surface areas is lower
than for high surface areas. As consequence, to obtain a similar reinforcement
with blacks of low surface areas the
crushed structure level has to be chosen
higher than it is necessary for blacks
with a higher surface area.
The tan d 60 8C behavior respectively the
heat build-up of filled rubber compounds
is exhibited in Fig. 12 resp. Fig. 13.
Fig. 12 depicts the response tan d 60 8C as
a function of surface area and structure. As
expected, the surface area has the main influence. Consequently, a black with a low
surface area and a high structure, in order
to have a sufficient reinforcing potential,
should give a low heat build-up in a Goodrich flexometer test. This in fact can be
seen in Fig. 13. Additionally it can be recognized that also the structure for low sur-
KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003
Fig. 12. The influence of specific surface area
and crushed DBP on the tan delta 60 8C behavior
Fig. 13. Heat build-up as a function of specific
surface area and crushed DBP
face area blacks has some influence on the
heat generation, but it is of minor importance.
The dynamic stiffness is a further important parameter for filled rubber compounds. Fig. 14 displays the dynamic stiffness E* measured at 60 8C as a function of
523
If a constant CTAB level is requested due
to the application, the only alternative to
decrease the electrical conductivity is to reduce the structure level. But it has to be
recognized that the influence of structure
on conductivity is less compared to the
soecific surface area.
References
Fig. 14. The dynamic modulus E*(60 8C) as a
function of specific surface area and crushed DBP
surface area and structure. Here, it can be
pointed out that the predominating factor
is given by the crushed DBP absorption.
Consequently, the selection of a low surface area of about 20 m2/g CTAB adsorption should have no negative impact on dynamic stiffness.
Conclusion
It was found that the secific surface area
has the major influence on the conductivity. On the other hand, to obtain a high reinforcing potential the structure level
should be chosen very high.
Consequently, low surface area blacks
with a high crushed DBP absorption
seem to be the right choice to meet the
requirement of low conductivity in combination with a high reinforcing potential.
A further advantage of such low surface
area blacks the low heat build-up, which
is also mainly ruled by the surface area.
[1] A. Voet, Rubber Chem. Technol. 54 (1981) 42.
[2] J.-B. Donnet, R. C.Bansal and M.-J. Wang, Carbon
Black Science and Technology, 2. Edition, Marcel
Dekker.
[3] W. F. Verhelst, K. G. Wolthuis, A. Voet, P. Ehrburger and J.-B. Donnet, Rubber Chem. Technol. 50
(1977) 735.
[4] M.-J., Wang, S. Wolff and E.-H. Tan, Rubber
Chem. Technol. 66 (1993) 178.
[5] A. I. Medalia, J. Colloid Interface Sci. 32 (1970)
115.
[6] J. Janzen, J. Appl. Phys. 46 (1975) 966.
[7] A. I. Medalia, Rubber Chem. Technol. 59 (1986)
432.
[8] A. I. Medalia, J. Coll. Interf. Sci., 32 (1970) 115.
[9] N. Probst, European Rubber J., Nov. (1984).
[10] H. Pawlowski and J. Dick, Rubber World 6 (1992)
35.
[11] J. Fröhlich, D. Luginsland and W. Niedermeier, paper No. 9, “Reinforcement mechanism in the rubber matrix by active fillers”, ACS Rubber Division,
Dallas, April (2000).
The authors
Dr. Werner Niedermeier and Dr. Joachim Fröhlich
are working in the product development group
of the Applied Technology Advanced Fillers at Degussa AG, Köln.
Corresponding author:
Dr. Werner Niedermeier
Degussa AG
FP-FA-AT1
Harry-Kloepfer-Str. 1
59997 Köln