st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Atmospheric plasma coating of tire reinforcing materials
F. Siffer1, J. Gillick1, D. Chandra1, L. Brace1, C. Vandenabeele2, R. Maurau2, S. Bulou2, P. Choquet2
1
2
The Goodyear Tire & Rubber Company, Akron, OH, United States
Centre de Recherche Public Gabriel Lippmann, Belvaux, Luxembourg
Abstract: Tires are complex composites made of rubber and reinforcing materials for which
excellent bonding is currently obtained. However, new bonding technologies are constantly
sought to expand beyond the constraints of existing art and achieve higher levels of key tire
properties such as durability and fuel efficiency. Plasma polymerized coatings were investigated as a new alternative for bonding rubber to galvanized steel cords.
Keywords: Atmospheric plasma polymerization, steel cord, film morphology, adhesion.
1. Introduction
Brass plated steel cords have been used in tires as reinforcing materials for several decades. The advantage of
brass lies in its ability to strongly bond to specific rubber
compounds without the need for prior surface modification. However, the presence of cobalt salts in the compound is required to maintain good brass-to-rubber adhesion over the service life of the tire. Due to the fact that
cobalt salts may become regulated in the future, new
technologies for bonding steel cords to rubber are being
explored. In this paper we present an atmospheric plasma
polymerization process for depositing thin coatings on
galvanized steel cords from chlorinated precursors. The
use of zinc as the plating element for steel cords combined with atmospheric plasma polymerization is a potential solution to go beyond the brass technology and its
constraints. Surprisingly, very few studies describe the
plasma polymerization of such precursors [1,2] while
plasma-assisted decomposition of chlorinated species is
largely described in the literature [3]. The goal of the present study is to understand how the reactor characteristics
and plasma processing conditions influence the growth of
the plasma polymerized film. Identification of desired
plasma coating properties is key for the development of
new rubber-to-metal adhesive systems.
2. Experimental details
An experimental tubular dielectric barrier discharge reactor (Fig. 1) was built to coat galvanized wires in a coaxial geometry.
The tubular prototype consists of a quartz tube. A spool
of galvanized wire is mounted on a let-off equipped with
a magnetic brake. The wire is strung through the quartz
tube and tied to a motorized wind-up unit for which the
winding speed can be controlled.
A Dielectric Barrier Discharge is ignited inside the tube
via a metallic tape wound around the quartz and connected to a high-voltage generator. The wire is connected to
the ground. The generator used in this study was purchased from AFS and features a frequency range comprised between 1 kHz and 100 kHz. Three different amplifiers are used to cover this frequency range. Plasma
polymerization is carried out using methylene Chloride
alone or blended with squalene. In the latter case, the
squalene-methylene chloride mixture is introduced into
the argon gas stream using an ultrasonic nebulizer from
Sonotek which allows control over the amount of precursor injected. In the case where methylene Chloride is used
alone, vapors of this precursor are simply carried into the
tubular reactor by using a gentle flow of carrier gas. Argon was used as the carrier gas in all cases. Pure argon is
also used as the ionization gas at flow rates comprised
between 4 L/min and 10 L/min. The ionization gas and
the precursor vapors are mixed at the inlet of the tubular
reactor. The inner diameter of the glass tube is 8 millimeters.
Wires were plasma coated in a static or dynamic mode.
SEM and XPS were used to analyze the resulting coating.
3. Results and discussion
One of the first studies aimed at determining the temperature seen by the wire inside the discharge zone when
varying the applied plasma power. To do that, a thermocouple junction surrounded by a steel tube was inserted
inside the quartz tube as depicted in Fig. 2 and Fig. 3.
Fig. 1 Tubular dielectric barrier discharge reactor
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig. 2 Experimental method for measuring the wire temperature
For this experiment, the discharge was ignited for 60
seconds and then cut-off to measure the time for the wire
to come back to room temperature. As expected, the temperature of the substrate rises with the plasma power. It is
important to have a good approximation of the temperature seen by the steel cord as exposure to high temperatures may have detrimental effects on the steel strength.
Additionally, the temperature also affects the coating
growth, composition and resulting morphology.
Fig. 5 presents the maximum temperatures obtained for
each plasma power when the discharge was prolonged
beyond 60 seconds. A maximum of 160ºC was obtained at
80 watts after 3 minutes of continuous exposure to the
plasma. However, the information to retain from Fig. 4
and Fig. 5 is that for short exposure times below 10 seconds, the power has little effects on the substrate temperature. Therefore, the substrate temperature during dynamic plasma coating of a steel cord should stay low due
to short residence times. On the other hand, for static experiments which will be discussed in the next section, the
effect of the temperature may not be negligible.
Fig. 3 View of the thermocouple inserted in the quartz tube
An Argon plasma was ignited at a frequency of 20 kHz
in the gap between the quartz tube and the steel tube
while the steel cord was maintained static inside the
quartz tube. Increasing plasma powers were applied and
the temperature recorded. Fig. 4 highlights the temperature profiles plotted versus time for increasing plasma
powers comprised between 10 W and 80 W.
Fig. 4 Plot of the wire temperature vs. time
Fig. 5 Maximum temperatures measured for each power
To further understand the effects of the plasma power
and temperature seen by the substrate, SEM images of
plasma coated galvanized filaments were recorded (Fig.
6). A 60/40 blend of methylene chloride / squalene was
used as the precursor. Plasma powers comprised between
5 W and 50 W were investigated. The deposition time was
set to 1 minute under static conditions. A substantial variation in film morphology can be clearly observed when
the power is increased. At 5 W, the coating appears very
smooth, of high quality and with very little surface defects
while the application of 20 W of power leads to a rough
and damaged coating. Buckling of the film occurs at 30 W
due to an increase of the film thickness combined with the
release of internal stress. Poor adhesion between the film
and the zinc plating may also favor the occurrence of this
phenomenon as the film may have grown too fast. Previous observations of plasma polymerized film buckling in
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
the literature were made and were found to be independent of the nature of the precursor. Cracks as well as powder particles appear at 50 W. The use of such a high power led to the formation of a highly cross-linked film which
was also exposed to a high temperature gradient. The reproducibility of the described morphologies was found to
be very good over time for different sample series.
observed in some cases between 10 and 25 W, evidencing
streamer impact propagation in a poorly conducting film.
This indicates that the surface roughness observed on
coatings obtained for a power of 20 W or higher is due to
streamers impacting and damaging the surface.
Additional experiments carried out at 20 W for respectively 30 s and 60 s of static polymerization highlight a
difference in film growth mechanism (Fig. 8). During the
initial 30 seconds of polymerization, a smooth, defect-free
coating is obtained like the one featured in Fig. 6 for a
power of 5 W. Beyond 30 seconds, a globular, cauliflower-like structure grows on top of the smooth film. Although this difference in film growth mechanism is not yet
fully understood, temperature effects combined with the
fact that the initial smooth film may act as an insulator are
most likely the root causes of such a change in film
growth mechanism.
Fig. 8 Influence of film thickness on growth mechanism
The film growth kinetics was also measured for a power of 20 W (Fig. 9). The coating grows linearly at a rate of
20 nm/s during the initial 30-second period while the erratic evolution into cauliflower-like structures does not
allow for precise thickness measurement beyond 30 s.
Fig. 6 Effect of plasma power on coating morphology
Fig. 9 Growth kinetics of CH2Cl2/Squalene polymerized films
Fig. 7 Lichtenberg figures obtained at 18 W
Additionally, Lichtenberg figures (Fig. 7) were also
XPS depth profiling was used to identify the composi-
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
tion and atomic wt% of the various elements across the
film thickness and interface. Fig. 10 shows depth profiles
respectively obtained for 5 W and 50 W. Pure methylene
chloride was as used as precursor. The static polymerization time was set to 10 seconds.
gon/oxygen plasma treatment prior to the coating deposition.
The XPS depth profile of the coating obtained at 50 W
shows similar features. The etching time to reach the zinc
is higher since the deposition rate increases with the power. The depth profile also shows slight variations in the
carbon/chlorine ratio throughout the film. Additionally,
the chlorine signal shows a hump at the interface with the
zinc oxide which is not seen for 5 W while the overlap
between the carbon, chlorine and zinc signals is more
pronounced compared to 5 W depth profile. This may be
due to initial etching of the zinc oxide by active chlorine
species [5] followed by implantation of chlorinated species in the roughness.
4. Conclusion
Initial work showed that plasma polymerized coatings
obtained from pure methylene chloride or squalene/methylene chloride blends were able to bond to rubber compounds. The study presented in this paper was
carried out to better understand the effect of plasma processing conditions on coating properties. Best coating
conditions were obtained for short deposition time and
low plasma power for which a thin and smooth film is
obtained that adheres well to the zinc substrate and to the
rubber. XPS depth profiles showed pronounced dechlorination of the precursor occurring during plasma
polymerization of methylene chloride even for plasma
powers as low as 5 Watts.
Fig. 10 XPS depth profiles of films polymerized at 5 W, 50 W
The depth profile of the coating obtained for a power of
5 W shows homogeneous composition throughout the
film with constant carbon/chlorine ratio. The film incorporates 15% of chlorine atoms which is substantially
lower compared to the initial 40% chlorine concentration
of methylene chloride. Indeed, the weakness of the carbon-chlorine bond as well as the eagerness of elemental
chlorine species to recombine into hydrochloric acid or
chlorine explains the poor incorporation of this element in
plasma polymerized coatings. The pronounced dechlorination observed in this study is surprising in light of results
obtained by Hubert et al. [4] on atmospheric plasma
polymerization
of
hexachlorobuta-1,3-diene
and
1,1,1,2-tetrachloroethane using a home-made dielectric
barrier discharge reactor. The XPS spectrum features carbon and chlorine signals decaying quickly with the onset
of the zinc and iron signals with a short overlap due to
substrate roughness. The substantial oxygen signal comes
from the wire surface cleaning performed with an ar-
5. References
[1] J. Csernica, D. Rhodes, J. Polym. Eng. 19, 1 (1999).
[2] R. Turri, C. Davanzo, W. Schreiner, J. Da Silva, M.
Appolinario, S. Durrant, Thin Solid Films, 520, 1442
(2011).
[3] G. Kamgang-Youbi, K. Poizot, F. Lemont, J. Hazard.
Mater. (2013).
[4] J. Hubert, C. Poleunis, A. Delcorte, P. Laha, J. Bossert,
S. Lambeets, A. Ozkan, P. Bertrand, H. Terryn, F. Reniers,
Polymer (2013).
[5] K. Nordheden, SPIE Proceedings, 5359, 228 (2004).