21st

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. Gillick

1, D. Chandra

1, L. Brace

1, C. Vandenabeele

2, R. Maurau

2, S. Bulou

2, P. Choquet

2

1The Goodyear Tire & Rubber Company, Akron, OH, United States

2Centre 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 investi-

gated 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 rein-

forcing 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 modifica-

tion. However, the presence of cobalt salts in the com-

pound is required to maintain good brass-to-rubber adhe-

sion 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 com-

bined with atmospheric plasma polymerization is a poten-

tial 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 pre-

sent 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 re-

actor (Fig. 1) was built to coat galvanized wires in a co-

axial 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 connect-

ed to a high-voltage generator. The wire is connected to

the ground. The generator used in this study was pur-

chased from AFS and features a frequency range com-

prised between 1 kHz and 100 kHz. Three different am-

plifiers 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 precur-

sor 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. Ar-

gon 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 millime-

ters.

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 tem-

perature seen by the wire inside the discharge zone when

varying the applied plasma power. To do that, a thermo-

couple 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

21st

International Symposium on Plasma Chemistry (ISPC 21)

Sunday 4 August – Friday 9 August 2013

Cairns Convention Centre, Queensland, Australia

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 tempera-

ture profiles plotted versus time for increasing plasma

powers comprised between 10 W and 80 W.

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 tem-

perature of the substrate rises with the plasma power. It is

important to have a good approximation of the tempera-

ture seen by the steel cord as exposure to high tempera-

tures 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 sec-

onds, the power has little effects on the substrate temper-

ature. Therefore, the substrate temperature during dy-

namic plasma coating of a steel cord should stay low due

to short residence times. On the other hand, for static ex-

periments which will be discussed in the next section, the

effect of the temperature may not be negligible.

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 vari-

ation 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. Previ-

ous observations of plasma polymerized film buckling in

Fig. 2 Experimental method for measuring the wire temperature

Fig. 3 View of the thermocouple inserted in the quartz tube

Fig. 4 Plot of the wire temperature vs. time

Fig. 5 Maximum temperatures measured for each power

21st

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 independ-

ent of the nature of the precursor. Cracks as well as pow-

der particles appear at 50 W. The use of such a high pow-

er led to the formation of a highly cross-linked film which

was also exposed to a high temperature gradient. The re-

producibility of the described morphologies was found to

be very good over time for different sample series.

Additionally, Lichtenberg figures (Fig. 7) were also

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 respec-

tively 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, cauliflow-

er-like structure grows on top of the smooth film. Alt-

hough 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.

The film growth kinetics was also measured for a pow-

er of 20 W (Fig. 9). The coating grows linearly at a rate of

20 nm/s during the initial 30-second period while the er-

ratic evolution into cauliflower-like structures does not

allow for precise thickness measurement beyond 30 s.

XPS depth profiling was used to identify the composi-

Fig. 6 Effect of plasma power on coating morphology

Fig. 7 Lichtenberg figures obtained at 18 W

Fig. 8 Influence of film thickness on growth mechanism

Fig. 9 Growth kinetics of CH2Cl2/Squalene polymerized films

21st

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 polymeriza-

tion time was set to 10 seconds.

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 incor-

porates 15% of chlorine atoms which is substantially

lower compared to the initial 40% chlorine concentration

of methylene chloride. Indeed, the weakness of the car-

bon-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 dechlorin-

ation 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 car-

bon 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-

gon/oxygen plasma treatment prior to the coating deposi-

tion.

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 pow-

er. 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 spe-

cies in the roughness.

4. Conclusion

Initial work showed that plasma polymerized coatings

obtained from pure methylene chloride or squa-

lene/methylene chloride blends were able to bond to rub-

ber compounds. The study presented in this paper was

carried out to better understand the effect of plasma pro-

cessing 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 dechlo-

rination of the precursor occurring during plasma

polymerization of methylene chloride even for plasma

powers as low as 5 Watts.

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).

Fig. 10 XPS depth profiles of films polymerized at 5 W, 50 W