ELASTOMERE UND KUNSTSTOFFE
ELASTOMERS AND PLASTICS
Rubber-metal bonding Electrochemistry NR SBR Brass Aluminium
Rubber-brass bonding during vulcanization has already been previously
discussed as a result of ionic reaction
mechanisms. If electrically charged
species (e. g. metal and sulfide ions)
are formed, the bonding process
should be influenced by an electric
field. An experimental set-up for the
application of a voltage during vulcanization and the preparation of test
specimens for bonding strength measurements is described. Electric conductivity is brought about by the carbon black component. Indeed, experiments with SBR mixtures/brass show
that an electric field influences the
bonding process and that under suitable conditions an improvement of
the bonding strength is observed. Unexpectedly rubber-metal bonding is
also observed with NR mixtures/aluminium (alloy with 5 % silicon) when a
voltage is applied.
Gummi-Metall-Bindung als
elektrochemischer Prozeû
Gummi-Metall-Bindung Elektrochemie NR SBR Messing Aluminium
Die Gummi-Messing-Bindung waÈhrend der Vulkanisation wurde schon
fruÈher als Resultat von ionischen Reaktionsmechanismen diskutiert. Wenn
elektrisch geladene Teilchen (z. B. Metall- und Sulfid-Ionen) gebildet werden, sollte der Bindungsprozeû durch
ein elektrisches Feld beeinfluût werden. Eine Versuchsanordnung fuÈr das
Anlegen einer Spannung waÈhrend der
Vulkanisation und die Herstellung von
PruÈfkoÈrpern fuÈr Messungen der Haftfestigkeit wird beschrieben. Die elektrische LeitfaÈhigkeit wird durch die
Ruû-Komponente bewirkt. Versuche
mit SBR-Mischungen/Messing zeigen
tatsaÈchlich einen Einfluû des elektrischen Feldes und unter geeigneten
Bedingungen eine Verbesserung der
Haftfestigkeit. Unerwarteterweise wird
eine Gummi-Metall-Bindung auch mit
NR-Mischungen/Aluminium (Legierung mit 5 % Silicium) beobachtet,
wenn eine elektrische Spannung angelegt wird.
Rubber-Metal Bonding as an
Electrochemical Process
K. Hummel, A. Filimonov, J. Hobisch and
F. J. Santos RodrõÂguez, Graz (Austria)
The bonding of rubber to metals is useful
in many industrial fields. For example, the
bonding of rubber to brass-coated steel
cord, carried out during vulcanization, is
important for car tires and pressure
hoses. The brass is used here as a
kind of bonding agent. The mechanism
of rubber-brass bonding has been subject of extensive research for decades,
for example by van Ooij et al. [1, 2].
The use of modern physical investigation methods, e. g. analytical electron
microscopy (AEM) [3 ± 5], have helped
elucidate the structure of the bonding
layer.
Rubber-brass bonding has already
been seen as an electrochemical process, the bonding layers (mainly containing non-stoichiometric copper sulfides
Cux S) being considered as semiconductors [6 ± 8]. A number of electrically
charged species were assumed, among
those the cations Cu‡ and Zn2‡ (from the
brass components), and anions such
as S2ÿ , R ÿ Sÿ and R ÿ Sÿ
x (from the
elemental sulfur of the rubber mixture, R ˆ substituent, ÿSÿ
x ˆ polysulfide
chain). The cations are thought to diffuse
in the direction of the rubber mixture and
the anions in the direction of the brass
surface. The basic idea of the present investigation is that ionization and ion
transport, and with that the bonding
strength, should be influenced by an
electric field. As far as known, experiments concerning this subject have not
been carried out to date.
The application of a voltage while curing a rubber mixture requires appropriate
vulcanization molds. Laboratory molds
must also produce formed pieces which
are suitable as test specimens for bonding strength measurements. Below, we
describe such a laboratory mold and
the procedure used for bonding strength
KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 11/2000
measurements. The experimental set-up
is described in detail in order to allow a
repetition of these experiments.
The compositions of the rubber mixtures are simplified in comparison with industrial mixtures. Some reasons, why a
scientific investigation of rubber-metal
bonding (in contrast to the optimization
of industrial processes) is only sensible
with mixtures containing a small number
of components, were given in [5]. The necessary electric conductivity of the rubber
mixtures is provided by the presence of
common types of carbon blacks.
Experimental
Vulcanization mold and
experimental set-up
Fig. 1 shows a cross-section of the split
cavity mold. The circular parts A and B
are made of aluminium. The rubber mixture is placed in cavity 1 between A and
B. The section of the test rod 2 (e. g. a
straight wire) in contact with the rubber
mixture is one of the electrodes. The
ring-shaped counter-electrode 3 has a
wire 3a fixed to it. The wires 4 and 5 connect 2 and 3a with the voltage source
using the couplings 6 and 7. The electrically insulating pieces 8 and 9 consist of
Teflon. The insulating foils 10 and 11 also
consist of Teflon. Mold part A has a ringshaped overflow channel 12 for the excess rubber mixture. In the following
Figs. 2 to 5, the designation of the components is the same as in Fig. 1.
Fig. 2 presents the inner surfaces of the
mold parts A and B. The electrodes are
not shown. The hole u is employed to insert the test rod 2. The wire 3a is inserted
into the hole v. The three threaded holes
w serve to open the mold by means of
651
Rubber-Metal Bonding as an . . .
Fig. 1. Mold for the application of a voltage to the vulcanization mixture, cross-section.
screw bolts after curing the rubber mixture.
Fig. 3 illustrates the crown-like shape
of the electrode 3. The gaps allow the
rubber mixture to flow across the electrode 3 at the start of the vulcanization
process before a coherent polymer network is formed.
Fig. 4 explains the principal set-up of
the complete arrangement for curing.
The mold parts A and B are placed between the additional aluminium parts C
and D which contain an electrically insulated hollow space to cover the wires 4
and 5, and the couplings 6 and 7. E
and F are the heating plates of a laboratory press.
The couplings 6 and 7 and the parts C
and D were only used for investigations
Fig. 2. Mold for the application of a voltage to the vulcanization mixture, inner surfaces of the mold parts A and B.
with metal rods (diameter > 0:5 mm).
For experiments with thinner wires, they
were not necessary. In this case, the
wires were longer and bent rectangularly
at the outer surface of the mold parts A
and B, covered with an electrically insulating heat-resistant material, and directed out of the mold through channels
cut into the outer surface of A and B.
Preparation of the test
specimens
The rubber mixtures were prepared on a
laboratory mill. The test rod 2 and the
counter-electrode 3 were inserted into
mold part A. In order to ensure the complete filling of the available space, a slight
excess of rubber mixture was cut into
small pieces and placed into cavity 1,
also between the electrodes 2 and 3.
The correct filling of the mold with the vulcanization mixture was important for reliable results.
Mold part B was put on part A. Subsequently parts A and B were pressed together. The couplings 6 and 7 (connected
to wires 4 and 5) were fixed on test rod 2
and wire 3a. Parts C and D were fitted as
depicted in Fig. 4. Parts A to D were
pressed together under a clamping pressure of 30 kN between the plates E and F
of the heating press. Then the voltage
was applied. The vulcanization was carried out with preset values of temperature
and reaction time.
It was found in preliminary experiments
that the effect of the electric current on
the bonding strength has an optimum
at a certain voltage and current density
(current/surface of the electrode) of the
test rod. It was also observed that an optimum polarity of the electrodes exists
(test rod anode or cathode, depending
on the metal).
Measurements
Fig. 3. Counterelectrode: (a) from
the side, (b) from
above.
652
A test specimen made with the set-up
described in Figs. 1 to 4 is shown in
Fig. 5 (a). The vulcanizate 13 with the
test rod 2 was obtained in the shape of
cavity 1. The counter-electrode 3 together with the wire 3a was not removed
from 13 for the determination of bonding
strength.
The main advantage of this type of test
specimen is that the samples can be pre-
KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 11/2000
Rubber-Metal Bonding as an . . .
Fig. 4. Application of a voltage to the vulcanization mixture, arrangement for curing.
pared and tested even with non-industrial
equipment and with rather small quantities of materials. It might be seen as disadvantage that the contact interface between vulcanizate and metal is small so
that the standard deviation of the average
of the bonding strength values is larger
than that obtained with the usual standardized bonding strength measurement
methods.
The principle of these measurements is
explained with Fig. 5(b). The test sample,
compare Fig. 5(a), is covered with two circular aluminium discs 14 and 15 which
are kept in their position at the levels m
and n with a special clamping device
which is fixed to a stable support. The
lower disc 14 has two holes x and y for
the rod 2 and the wire 3a, respectively.
The upper disc 15 has a hole z for rod
2. The diameters of the holes x and z
should be large enough to avoid friction
between the rod 2 and the discs 14
and 15. The diameter of the hole z exerts
a certain influence on the results of the
bonding strength measurements and
should be well-defined. The upper end
of rod 2 is connected to a tension testing
machine by the clamping device 16. The
arrow d shows the direction of tension.
Before measurement, the specimens
were stored for about 20 h at 20 8C. Subsequently, the test rod 2 was pulled out at
a rate of 100 mm/min. The strength p at
the moment of tearing out was measured
in N. The interface f between vulcanizate
and test rod gave the bonding strength
b ˆ p=f in MPa.
Fig. 5. Measurement of the bonding strength with the test specimens made in the mold in Figs. 1 to 4: (a) test specimen, (b) principle
of the measurements.
Rubber mixes and test specimens
Experiments concerning rubber-brass
bonding were carried out with styrenebutadiene rubber (SBR). Because the
SBR 1712 used already contained 37.5
parts of mineral oil per 100 g of polymer,
all further additives were calculated for
137.5 parts by weight of SBR 1712,
see Tab. 1. The brass-coated steel cord
was a standard product commonly
used in rubber industry and employed
as received. It had a diameter of
0.25 mm. The length of the wire in contact with the rubber mixture was
5.2 mm. Accordingly contact interface
was 4:08 mm2 . In bonding strength measurements, the hole z in disc 15, see
Fig. 5(b), had a diameter of 1.0 mm.
Experiments concerning rubber-aluminium bonding were carried out with natural rubber (NR). The mixture given in
Tab. 1 uses NR-RSS3. It was shown in
preliminary experiments that the tear resistence of wires consisting of pure aluminium was lower than the bonding
strength achieved with the application
of a voltage. Therefore experiments
with rods consisting of an aluminium alloy
(ªAlSi5º) containing 5 % silicon, < 0:4 %
iron and < 0:16 % titanium are described. The rods were cleaned by pretreatment with emery cloth and treatment
with an aqueous solution of sodium hydroxide (3 % w/v) for five minutes at
room temperature, followed by washing
with water and acetone. The AlSi5 rods
had a diameter of 1.2 mm. The part of
KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 11/2000
the rod in contact with the rubber mixture
was 4.0 mm long. The contact interface
was ca. 16 mm2 . In bonding strength
measurements, the hole z in the disc
15 in Fig. 5(b) had a diameter of 2.0 mm.
Results and discussion
The examples 1 to 6 listed in Tab. 1 were
investigated. Each series of experiments
was carried out with a sufficiently large
number of test specimens, prepared under identical conditions, to obtain a reliable average (arithmetic mean). The results are given in Tab. 2.
Bonding of styrene-butadiene
rubber (SBR) vulcanizates to
brass-coated steel
In example 1, experiments were carried
out with 40 samples prepared under
identical conditions to obtain the average. The cure time was the rheometrically
determined tc 90 time [9] of 37 min at
150 8C. In the first series of experiments,
the samples were cured without the
application of a voltage. The result was
a bonding strength b with an average
of 4.3 MPa and a standard deviation of
s ˆ 2:62 MPa. For brass-coated steel,
this is a rather low bonding strength. Rupture occurred between the rubber and
brass-coated steel interface and not in
the rubber phase which is the case
with satisfactory bonding. In a second
series, a DC voltage of 6 V was applied.
Better results were obtained when the
653
Rubber-Metal Bonding as an . . .
Table 1. Rubber mixes (parts by weight)
example
1
2
3
4
a
b
c
rubber
carbon black
SBR 1712
(137.5)
ª
NR-RSS3
(100.0)
ª
5
ª
6
ª
N 234
(84.7)
N 375
(84.7)
N 115
(50.0)
N 234
(50.0)
N 121
(50.0)
CD 2041
(50.0)
softener
sulfur
accelerator
a
mineral oil
(24.2)
ª
S, powder
(2.06)
ª
DCBSC
(2.06)
ª
mineral oilb
(10.0)
ª
S, powder
(6.40)
ª
DCBSC
(2.40)
ª
ª
ª
ª
ª
ª
ª
aromatic
naphthenic
N,N-dicyclohexyl-2-benzothiazolsulfenamide
test rod was the anode as opposed to
being the cathode. A maximum current
of ca. 200 mA was observed. A bonding
strength b with the average of 13.74
MPa and a standard deviation of
s ˆ 1:36 MPa were found under these
conditions. Rupture occurred within the
rubber phase and the wire remained
coated with rubber.
For example 2, the experimental procedure corresponded to that in example
1. The cure time tc 90 was 35 min at
150 8C. The first set of samples were
again cured without the application of a
voltage. The average value of the bonding
strength b was 5.07 MPa. The standard
deviation was s ˆ 1:29 MPa. In a second
set of experiments, a DC voltage of
10 V was applied which corresponded
to a maximum current of ca. 50 mA.
The average bonding strength b was
9.75 MPa with a standard deviations
s ˆ 2:64 MPa.
The experiments confirm that the rubber-brass bonding under consideration is
influenced by an electric field. A possible
consequence of this could be changes in
the bonding layer structures. However,
tentative investigation of bonding to
brass-coated steel cord by means of
AEM did not give unequivocal evidence
of differences in the bonding layers obtained in the absence (i) and presence
(ii) of an electric field. Bonding layers
were extracted parallel to the brass surface by chemical etching as described
previously [4]. AEM showed the usual
phases of non-stoichiometric copper sulfides Cux S and no striking differences between (i) and (ii) were found.
Bonding of natural rubber (NR)
vulcanizates to an aluminium alloy
with 5 % Si
For example 3, the cure time tc 90 was 7
to 8 min at 150 8C. Considering the time
for heating up the model to 150 8C at the
beginning of the reaction, a somewhat
longer cure time of 10 min was chosen.
The 27 samples cured without the application of a voltage had a bonding
Table 2. Rubber-metal bonding, effect of an electric fielda
a
b
c
example
metal
bonding strength (MPa)
without voltage
with voltage
1
2
3
4
5
6
brassb
ª
AlSi5c
ª
ª
ª
4.30
5.07
1.48
1.36
1.24
1.35
13.74
9.75
7.50
5.56
5.55
6.98
For the composition of the reaction mixtures see Table 1. Voltage values, standard deviation of the
bonding strength and other experimental details see text.
brass-coated steel cord
aluminium with 5% silicon
654
strength with an average value of
1.48 MPa.
The
standard
deviation
was
s ˆ 0:91 MPa. In a comparative series,
a DC voltage of 8 V was applied. In this
case the test rod was the cathode as
this polarity gave the best results. The
maximum current was between 100
and 200 mA. After 35 measurements,
the average of bonding strength b was
found to be 7.50 MPa with a standard deviation s ˆ 1:48 MPa. There was almost
no bonding of the counter-electrode 3,
also consisting of aluminium, to the vulcanizate. This can be explained with
the lower current density at electrode 3,
which has a much larger surface area
than electrode 1.
For examples 4 to 6, the experimental
procedure was identical with example 3
except that voltages up to 14 V were applied. All the results indicated that in all
cases a significant increase in bonding
strength was obtained after the application of a voltage during vulcanization.
A preliminary investigation by means of
transmission electron microscopy (TEM)
showed no bonding layer comparable
to the copper sulfide layers found in brass
bonding. A formation of Al2 S3 from Al and
S is only known for high temperature reactions.
Concluding remarks
The principal result of these investigations
is that the bonding strength obtained in
rubber-metal bonding can be improved
(in the case of brass) or even made possible (in the case of aluminium) by applying an electric field during vulcanization
under suitable conditions. Attempts to
connect this electrochemical rubber-metal bonding with possible changes in
bonding layers were unsuccessful. The
question is whether or not these results
are in line with the present knowledge
of rubber-metal bonding.
Most models of rubber-brass bonding
consider three phases: rubber bulk,
ªbonding layerº, and brass bulk. The
bonding layer (e. g. with a total thickness
> 50 nm) contains various copper sulfides and zinc sulfide. Unsatisfactory
bonding has been thought to occur if
this bonding layer is missing or is too thick
(e. g. > 500 nm). Investigations of crosssections perpendicular to the brass surface have completed this picture [4, 5].
KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 11/2000
Rubber-Metal Bonding as an . . .
The chemical composition of small areas
of the cross-sections was determined by
means of energy-dispersive X-ray spectroscopy (EDXS). Crystal structures
were identified by means of selected
area electron diffraction (SAED). Further
investigation methods such as electron energy-loss spectrometry (EELS),
convergent beam electron diffraction
(CBED), and electron spectroscopic imaging (ESI) were applied. Various morphological structures of the bonding layers
could be distinguished. The simplest
structure of a bonding layer cross-section, found in transmission electron microscopy (TEM) investigations [5], is
shown schematically in Fig. 6. The rubber
bulk is seen as the bright area R and the
brass bulk as the dark area B. Between R
and B there are two sublayers; neighboring R a dark one (termed D), and neighboring B a narrow bright one (termed E); d
is the thickness of the sublayer. The combination of D and E has been observed in
most cross-sections. D contains copper
sulfides and zinc sulfide. E contains
more zinc than D, and the copper content
is lower than in D (both elements in the
form of sulfides and oxides). It was shown
that the rubber phase does not end between R and D (level g) but nearly reaches
to the brass surface (level h). The rubber
near the brass surface contains polysulfide chains, and is thought to be a kind of
ªcementº for all sublayers.
A suitable model of rubber-metal
bonding observed after the application
References
[1] W.J. van Ooij, Rubber Chem. Technol., 51 (1978)
52.
[2] W.J. van Ooij, Giridhar and J.H. Ahn, Kautsch.
Gummi Kunstst., 44 (1991) 348.
[3] T. Kretzschmar, F. Hofer, K. Hummel and F. Sommer, Kautsch. Gummi Kunstst., 46 (1993) 710.
[4] F. Hofer, G. Grubbauer, K. Hummel and T.
Kretzschmar, J. Adhesion Sci. Technol., 10
(1996) 473.
[5] K. Hummel, F. Hofer and T. Kretzschmar, J. Adhesion Sci. Technol., 10 (1996) 461.
[6] G. Haemers, Adhesion (Barking, England), 4
(1980) 175.
[7] G. Haemers, Rubber World, Sept. 1980, 26.
[8] W.J. van Ooij, Rubber Chem. Technol., 57 (1984)
421.
[9] DIN 53529,2; DIN-VDE-Taschenbuch Bd. 47,
Kautschuk u. Elastomere 1, Beuth Verlag
GmbH, Berlin, KoÈln 1988.
Acknowledgements
Fig. 6. Schematic drawing of a simple bonding layer in rubber-brass bonding cross-section perpendicular to the brass surface [5].
R ˆ rubber; D, E ˆ sublayers; B ˆ brass;
g, h ˆ interfaces.
of an electric field is that this bonding is
attributable to changes in interface h between rubber and metal, and that the
changed areas have a size below the resolution of routine TEM and AEM, this
means at a molecular scale. The practically unchanged Cux S layers in the rubber-brass bonding case, and the occurrence of rubber aluminium bonding
(where sulfide layers of the Cux S type
are not possible) are consistent with
this model.
KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 11/2000
We thank Prof. Dr. F. Sommer (Semperit Technische
Produkte GmbH, Wimpassing, Austria) for support,
G. Lehr (Technical University Graz) for his assistance
in the construction of vulcanization molds and measurement devices, and Dr. V. Nigmatullin (presently at
Technical University Graz) for carrying out measurements by means of electron microscopy.
The authors
The authors performed these investigations at the Institute for Chemical Technology of Organic Materials
(ICTOS) of the Technical University Graz. Dr. Klaus
Hummel is em.o.Univ.-Prof., Dr. Andrej Filimonov
(Moscow) was a guest scientist at ICTOS, Ing. Josefine Hobisch is member of the ICTOS staff, Dr. Francisco J. Santos RodrõÂguez finished his chemistry studies at the Industrial University of Santander (Colombia) and his doctoral thesis at ICTOS.
Corresponding author
Dr. K. Hummel
Institut fuÈr Chemische Technologie Organischer
Stoffe der Technischen UniversitaÈt Graz
Stremayrgasse 16/1
A-8010 Graz
655