22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
A contribution to understanding the mechanisms involved during plasma
exposure of non-vulcanized polybutadiene rubber - Towards new adhesive
performances
A. Henry1, C. Noel2, M.F. Vallat1, T. Belmonte2 and V. Roucoules1
1
Institut de Science des Matériaux de Mulhouse, UMR 7361 CNRS - Université de Haute Alsace, FR-68057 Mulhouse,
France
2
Institut Jean Lamour, UMR 7198 CNRS - Université de Lorraine, FR-54011 Nancy, France
Abstract: Plasma treatments can be used to modify surface properties of vulcanized
rubbers. Efforts have been made in the understanding of the complexity of plasma-surface
interaction mechanisms. Most of the time, the complexity of the composition of vulcanized
rubber (sulfur, additives, vulcanizing agents, ...) does not allow any identification of the
role of each component. In this work we propose to work on pure non-vulcanized rubber to
understand the impact of plasma exposure on the polymeric chains taken separately.
Keywords: rubber, polybutadiene, plasma treatment, surface modifications, autohesion
1. Introduction
Performance of adhesive bonding of rubbers is decisive
in many applications [1]. The strength and quality of
such adhesive-bonded joints depend on crucial parameters
such as compatibility (for chain interdiffusion in polymerpolymer assemblies), interactions and co-crosslinking. In
some cases, surface treatments are used in order to
increase surface reactivity.
Among them, plasma
treatment has become a powerful candidate as it combines
high chemical reactivity with low operational costs, in
environmentally friendly processes.
Plasma treatment has usually low operational costs. It
has been intensively applied for surface modification of
vulcanized rubbers [1, 2]. Almost no studies have been
dedicated to plasma treatment of non-vulcanized rubbers.
The plasma process results in physical and/or chemical
modification of the first molecular layers of the surface
while retaining bulk properties. During plasma exposure,
the surface is driven away from its thermodynamic
equilibrium and after plasma treatment, the modified
surface reconstructs (e.g., chain reorganization, migration
of additives, …) in order to return to an equilibrium state
[4]. This dynamics is strongly dependent on the nature
and number of additives in the formulations. The role of
each additive during plasma exposure is poorly
understood. It is also admitted that surface crosslinking
occurs easily on rubber surfaces exposed to plasmas. This
impacts the adhesive properties of the rubbers because it
minimizes chain interdiffusion.
In this context, determining the key plasma parameters
that have a significant effect on the surface properties is a
prerequisite for further process control.
To limit the complexity of the study, we concentrated
our efforts on non-vulcanized filler-free polybutadiene
rubber (BR) which has been used for the first time as a
rubber model. The effect of three main parameters that
directly impact the amount of energy and the nature of the
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excited species in the plasma phase (i.e., the R.F. source
power, the exposure time and the distance between the
rubber samples and the plasma) was analyzed. Optical
Emission Spectroscopy was used to characterize the
plasma phase. Surface modifications were investigated
by wettability measurements and X-ray Photoelectron
Spectroscopy.
Surface aging was examined under
ambient and inert atmosphere at different temperatures.
Finally, the performances of adhesive bonding of plasma
treated rubbers were evaluated by tack measurements.
2. Materials and Methods
2.1. Material and sample preparation
Commercial
Polybutadiene
(BR)
(1.9%
of
1,2 polybutadiene, 1.1% of 1,4 trans polybutadiene and
97% of 1,4 cis polybutadiene) was used and molded under
pressure to get 2 mm thick sheets.
2.2. Plasma treatment
A low pressure radio-frequency (R.F) air plasma
treatment was used to modify the surface of the
unvulcanized BR films. Plasma processing was carried
out in a home-built R.F. plasma reactor. The plasma
chamber consisted of a glass tube (6 cm in diameter,
680 cm3 in volume) coupled with an externally wound
copper coil (4 mm diameter, 5 turns) and topping a 20 cm
cylinder glass vessel. With this design, modifying the
distance between the sample and the plasma was possible
in a large range. An L-C matching network was used to
couple the 13.56 MHz R.F. power supply with the
partially ionized gas load and minimize the reflected
power. The air flow was 1.776 cm3/min, the base
pressure was 8×10-3 mbar and the work pressure was
2×10-1 mbar. The three main parameters studied in this
work were the RF source power (5 W and 60 W), the
exposure time (30 s and 5 min) and the distance between
the rubber sample and the plasma (position B, 20 cm from
1
the bottom of the coil which is somehow the plasma edge
or position C, 34 cm, Fig. 1).
2.6. Tack Measurements
Adhesion strength was determined through tack
measurements (autohesion) for short contact times under
controlled atmosphere (20 °C and RH of 50%) [6]. The
force applied during contact was 20 N and contact times
were 1 s, 5 s and 100 s.
Gauge
r-f generator
Matching network
Copper coil
Plasma
Pump
Liquid
nitrogen
Air
Position B
Glass plate
(FWHM) using CASAXPS software. XPS analyses were
performed before and after aging.
Sample
Position C
3. Results and Discussion
Fig. 1 shows a schematic representation of our homebuilt plasma reactor. Polybutadiene films were placed in
position B or in position C. Molecules contained in the air
are introduced into the vacuum chamber and dissociated
in the plasma. This gives rise to emission of species
coming from N 2 , O 2 and H 2 O that are listed in Table 1.
Atomic oxygen emission is produced by electron-impact
excitation from ground state. The H-α line at 656 nm and
H-β line at 486 nm confirms the presence of water.
Fig. 1. Schematic representation of the plasma reactor
Table 1. List of emitting species identified in the air
plasma discharge.
2.3. Optical Emission Spectroscopy (OES)
Optical emission spectroscopy measurements of the
plasma phase were carried out to determine the various
emitting species in the plasma phase. The discharge light
was collected by an optical fibre connected to a Jobin
Yvon TRIAX550
spectrometer
equipped
with
3 diffraction gratings (1800, 1200 and 100 lines/mm) and
an ICCD detector [5].
Emitting species
N 2 (C3Π+ u - B3Π+ g )
N 2 (B3Π g -A3Σ u +)
N+ 2 (B2Σ u +-X 2 Σ g +)
OI
Hα, Hβ
CN(B2Σ-X2Σ)
Na I
2.4. Contact Angle Measurements
Contact angle measurements were carried out using a
contact angle DSA100 goniometer (Krüss GmbH) at
room temperature. Static contact angle measurements
were determined by the sessile drop method using
distilled water drops of 2 µl. Measurements were taken at
different times after plasma treatment (0, 2, 4 and
72 hours of aging) and under different conditions of
storage (ambient atmosphere or inert N 2 atmosphere).
NO 2
2.5 X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) spectra were
recorded with a VG SCIENTA SES 2002 spectrometer
equipped with a concentric hemispheric analyzer. The
incident radiation was generated by a monochromatic
Al Kα X-ray source (1486.6 eV) operating at 420 W
(14 kV; 30 mA). Photo-emitted electrons were collected
at a take-off angle of 90° from the substrate, with electron
detection in the constant analyzer energy mode. Survey
spectra were recorded with a pass energy of 500eV while
high-resolution spectra (C1s, O1s and N1s) were recorded
with a pass energy of 100 eV. Charging effects were
compensated by the usage of a flood gun. Peak fitting
was carried out with mixed Gaussian–Lorentzian (30%)
components with equal full-width-at-half-maximum
2
Wavelength (nm)
337, 358, 360, 406
661, 670, 750
391
436, 777, 844
656, 486
388
588, 598
Deviation of the baseline
between 400 and 700 nm at
low discharge power
Exposure to low-pressure air plasma readily undergoes
chain scissions and formation of free radicals at the
surface of the BR films. These radicals react mainly with
plasma reactive species and oxygen of the atmosphere
(after removal of the samples from the plasma chamber)
to form an oxidized and hydrophilic layer. They can also
react together, inducing cross-linking at the topmost
surface layer.
Contact angle measurements and X-ray photoelectron
spectroscopy analyses confirmed the aforementioned
explanations. The values of the contact angles decreased
after plasma treatments (see Fig. 2), indicating an increase
in hydrophilicity of treated surfaces [7]. Fig. 3 shows the
deconvolution of C1s XPS signals and reveals the
formation of COOH, C=O and COR moieties on BR
surface after treatment. The final contact angle values
and the final composition of the BR topmost layer depend
strongly on the discharge power.
The whole observations can be explained by the
competition between oxidation processes and crosslinking
phenomena at the topmost surface of the BR films during
plasma exposure. The balance between oxidation and
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Fig. 2. Contact angle values of BR surfaces obtained
before and after plasma exposure.
BR 1s
BR 5s
BR 100s
400
300
200
100
C-OR
Fig. 2. XPS C1s envelope of BR surface a) before plasma
exposure and b) after 60W 5min B plasma exposure.
The result of this balance affects the autohesion
properties as shown in Fig. 4. Both maximum strength
and energy of adhesion depend on the plasma parameters
with promising results obtained in the case of higher
plasma-sample distance. Details i) on the mechanisms
involved during plasma-rubber interaction, and ii) on the
role of additives in the rubber formulation will be given
during the oral presentation.
5W 30s C
5W 30s B
5W 5min C
5W 5min B
60W 30s C
60W 30s B
60W 5min C
0
b)
COOH C=O
b)
500
Reference
a)
Adhesion energy Wadh (N)
600
60W 5min B
Water contact angle value (°)
Position B
Position C
a)
Strengthmax (N)
crosslinking processes can be finely adjusted by playing
with the plasma power and the plasma-sample distance.
This has also an impact on ageing processes which are
mainly driven by temperature-dependent phenomena.
Fig. 3. Tack measurements (autohesion) for BR surfaces
for all plasma exposures : a) Maximum strength and b)
adhesion energy (Wadh) for 1s of contact (black), 5s of
contact (white) and 100s of contact (grey)
5. References
[1] I. Rezaeian, P. Zahedi and A. Rezaeian. J. Adhesion
Sci. Technol., 26, 721 (2012)
[2] M.M. Pastor-Blas, J.M. Martin-Martinez and
J.G. Dillard. Surf Interface Anal. 26, 385 (1998)
[3] E.M.J. Liston. J. Adhesion, 30, 199 (1989)
[4] M. Mortazavi and M. Nosonovsky. Appl. Surf. Sci.,
258, 6876 (2012)
[5] M. Mafra, T. Belmonte, F. Poncin-Epaillard,
A.S. Da Silva Sobrinho and A. Maliska. Plasma
Chem. Plasma Process., 28, 495 (2008)
[6] M. Mikrut, J.W.M. Noordermeer and G. Verbeek.
J. Appl. Polymer Sci., 114, 1357 (2009)
[7] D. Bodas and C. Khan-Malek. Sensor Actuator B:
Chem., 123, 368 (2006)
4. Conclusion
Low-pressure air plasma has been successfully used to
improve autohesion between two non-vulcanized BR
films. The key plasma parameters acting significantly on
the surface of the non-vulcanized polybutadiene used as a
rubber model have been determined and the mechanism
involved in the performance of bonding has been
elucidated.
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