st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Plasma treatment of polypropylene containing different additives
H. Behm1, H. Bahre2, K. Bahroun1, M. Böke2, R. Dahlmann1, Ch. Hopmann1, J. Winter2
1
Institute of Plastics Processing (IKV) at RWTH Aachen University, Aachen, Germany
2
Institute for experimental physics II, Ruhr-Universität Bochum, Bochum, Germany
Abstract: Plastics, especially polyolefins contain a number of additives ensuring a large
production window, a long product life time and favorable mechanical properties. Regarding a
functionalization process in plasmas after production, some additives influence the resulting
properties. The aim of this study is to evaluate the influence of additives in injection molded PP
samples on the properties after functionalization in low pressure plasmas.
Keywords: additives, adhesion, polymer, polypropylene, plasma treatment
1. Introduction
In plastics processing plasmas are commonly used to
modify surface properties of thermoplastic parts. There is
a wide range of surface modification possibilities. Next to
cleaning, etching and activation processes the deposition
of highly functional polymer films is achievable. Cleaning
and activation processes are used e.g. to enhance the
printability or wettability [1, 2]. Etching can be used to
implement micro structures or reduce adhesion for
example of bulk goods. Deposition processes are used in
order to create scratch resistant surfaces, reduce friction
or to enhance permeation barriers [3-5]. In most cases,
polymers are treated or coated in plasmas regardless of
their specific material type or composition. In industrial
application, all polymer materials used for injection
molding and extrusion processes contain additives. There
is a large variety of additives, including those necessary
for the production processes (e.g. slip agents, antioxidants, and lubricants) and those enhancing i.e.
mechanical properties, chemical resistance or degradation
processes caused by radiation. Some additives migrate to
the surface after production of a plastic part; others stay in
the bulk material. Studies have shown that certain
additives are likely to influence the surface properties
after functionalization [6, 7].
In this study, injection molded polypropylene samples
containing different additives are examined. Samples are
treated in oxygen and argon plasmas to enhance coating
adhesion of HMDSO-based thin films. The aim is to
identify the influence of typically used additives
regarding the resulting surface properties after plasmatreatment as well as coating adhesion.
2. Experimental Details
The injection molding process is a discontinuous
process. The polymeric material is fed in form of pellets
into a plasticizing unit in which they are melted due to
friction and heat transfer. If additives essential for an
application are not included in the chosen resin, a so
called master batch can be used. Master batches are also
distributed in form of pellets containing the same or
similar base polymer as the used resin and additional
additives. Before the process, they are mixed with the
base material in a defined, calculated ratio.
The base material studied in the experiments is an
isotactic polypropylene (PP, HD601CF, Borealis
Polyolefine GmbH, Linz, Austria) which is typically used
for extrusion purposes. It already contains small amounts
of different additives (e.g. anti-oxidants, anti-block
particles). Master batches are produced using a double
screw extruder (ZSK 26 Mc, Coperion GmbH, Stuttgart,
Germany). The investigated additives are listed in table 1.
Table 1. Additive types and used amounts.
Additive
(CAS number)
application
content [wt%]
Crodamide ER
(112-84-5)
lubricant
0.3
ADK Stab 2112
(6683-19-8)
anti-oxidant
0.2
Chimasorb 944 FDL
(71878-19-8)
hindered amine
light stabilizer
0.5
In an injection molding process, test bars according to
DIN EN ISO 527 are produced on an Arburg Allrounder
370 A 600-170 ALLDRIVE (Arburg GmbH + Co KG,
Loßburg, Germany) with an additive content shown in
table 1. Melt temperature is set to 240 °C and mold
temperature is kept at 65 °C. The chosen contents were
discussed with Borealis before production and set to the
upper limit of commonly used levels.
For the plasma treatment and coating, a laboratory
reactor with microwave (MW) excitation is used. A
substrate holder (300 x 300 mm²) is positioned vertically
in the chamber with a distance of 135 mm from an array
of 4 duo plasmalines described in [8] driven at 2.45 GHz.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
After injection molding, the low molecular amides
(Crodamide) migrate to the surface and form a thin film
[6]. Next to carbon from the PP, oxygen and nitrogen are
visible in the XPS survey (Fig.1).
Pre-treatment is done in pulsed MW mode with oxygen
and argon as process gases. The electron densities for the
pre-treatment plasmas are measured to 9x1016 m-3 with a
plasma absorption probe described in [9]. The potential
drop between plasma and floating potential is 8 V
according to Langmuir probe [10] measurements. Pulse
duration is set to 44 ms with a pulse on-time of 4 ms.
For the application of the plasma polymeric coating,
hexamethyl disiloxane (HMDSO) is used as monomer
gas. Similar to the pre-treatment process, pulsed MW
mode is used for plasma excitation. Pulse duration is
102 ms while the pulse on-time is 2 ms. The resulting
silicon organic (SiOCH) coatings are commonly used as
adhesion promoting layer for silicon oxide (SiOx) barrier
coatings on PP [11].
Static contact angles of water and diiodomethane are
measured with a drop volume of 1 µl using an OCA 20
from Data Physics Corporation, San Jose, CA, USA. The
shown mean values are based on a measurement of
10 drops using the tangent leaning method.
1.2
1.0
0.8
0.6
N1s
0.2
0.0
600
500
400
binding energy [eV]
300
200
Fig.1 XPS survey of a sample without additional additive and a
sample with 0.3 % Crodamide ER.
Comparing
the
chemical
structure
(CH3(CH2)7CH=CH(CH2)11CONH2) of Crodamide with
the spectra, gives rise to the assumption, that amides are
present on the surface. Analyzing the C1s peak of the
different samples, a difference is apparent between both
samples (Fig.2).
1.2
HD601CF
1.0
+0.3 % Crodamide
0.8
0.6
0.4
0.2
0.0
290
288
286
binding energy [eV]
284
282
Fig.2 C1s peak of a sample without additional additive and a
sample with 0.3 % Crodamide ER.
Figures 3 and 4 show water and diiodomethane contact
angles on samples treated in argon (Fig.3) and oxygen
plasmas (Fig.4).
120
water (HD601CF)
diiodomethane (HD601CF)
100
contact angle [°]
3. Results & Discussion
All untreated sample types are characterized using
FTIR (attenuated total reflection, ATR-Mode, 2-3 µm
depth) and XPS. Because of the small additive amount in
the samples, FTIR results show no difference in the
spectra.
0.4
O1s
intensity [-]
HD601CF
Surface chemistry is analyzed with X-ray photoelectron
spectroscopy (XPS). Measurement device is a M-Probe
from Surface Science with a monochromatized AlKα
radiation at 1486.6 eV.
The bond strength between PP and the applied CVDcoatings is measured in pull off tests based on DIN EN
ISO 4624. The tests are carried out using a tensile test
device (Zwick Z010, Zwick GmbH & Co. KG, Ulm,
Germany) with a 10 kN load cell. The coated specimens
are glued in between two cylindrical rods with a diameter
of 20 mm on the coated side and 30 mm on the uncoated
side and pulled off with a constant load increase of
0.5 MPa/s. The adhesive used on the coating side is the
two component epoxy system Loctite M-21HP (Henkel
AG & Co. KGaA, Düsseldorf, Germany). On the PP side,
an aliphatic amine primer (Loctite 770) is applied before
gluing with Loctite 401 (ethyl cyanoacrylate, cured at
23 °C for >18 h). One trial always consists of 10 samples.
Time between coating and analysis is kept similar.
C1s
+0.3 % Crodamide
Intensity [-]
Non-layer forming gases (Ar, O2) are fed through a gas
distribution system behind the plasmalines. The layer
forming gas is fed between the plasmaline array and
substrate holder using a ring shaped shower head.
water (+0.5 % Chimasorb)
diiodomethane (+0.5 % Chimasorb)
80
60
40
20
0
0
1
2
3
treatment time [s]
4
Fig.3 Contact angles on samples treated in argon plasma.
5
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Plasma treatment time varies between one pulse (4 ms)
and 5 s. In argon plasma a boundary value of about 60° is
reached after 2 s for the water contact angle on reference
samples as well as samples containing additional
Chimasorb. Diiodomethane contact angles show a slight
increase from 0.2 ms to 5 s while they stay almost
constant on the reference samples (HD601CF).
Using the Bohm-flux Γ = nvB with the electron density
measured in the sheath region results in an ion flux to the
surface in the order of 1020 m−2 s−1. Given that a
monolayer of PP has roughly 1019 m−2 atoms, it is clear
that such a significant change is possible even after a 1 s
treatment.
120
120
untreated
0.5 s oxygen
2 s argon
100
80
60
40
20
0
Fig.5 Water contact angles on untreated and treated samples.
water (HD601CF)
water (+0.5 % Chimasorb)
diiodomethane (+0.5 % Chimasorb)
80
60
40
20
0
0
1
2
3
treatment time [s]
4
5
Fig.4 Contact angles on samples treated in oxygen plasma.
The behavior of samples treated in oxygen plasmas is
deviating from the ones treated in argon. Because of the
high chemical reactivity of oxygen plasmas the measured
contact angles are low even after short treatment times. A
minimum (reference: 59.17°±0.9°) can be identified at
0.2-0.5 s treatment time for the water contact angle. An
increasing difference between water contact angles on
samples with Chimasorb and reference samples can be
noted for longer treatments. This disparity might
correspond to the stabilizing effect of the light stabilizer.
It will be addressed in further XPS studies.
Changes in diiodomethane contact angles are small
over treatment time. Therefore, a distinct change in
topography is unlikely while the polar component of the
surface energy changes throughout the process.
The applied argon treatments always show the same
effect regardless of the presence of a further additive
(Fig.5).
Examining the influence of oxygen plasmas on the
wetting behavior of samples containing additional ADK
Stab 2112 and Crodamide ER, differences can only be
seen for Crodamide and the already discussed Chimasorb
(Fig. 5). A 0.5 s pre-treatment in oxygen plasma results in
a water contact angle of 74.4°±0,9° (reference: 59,8°±1°).
Because of the Crodamide on the surface, it can be
assumed, that the test liquid forms a solution with the
amides producing distorted values. Nonetheless, a
difference in adhesion force of subsequently applied
SiOCH coatings can be identified.
Pull-Off tests of samples coated subsequently to the
pre-treatment reveal a positive influence of an oxygen
treatment for short times between 4 ms (1 pulse) and 0,5 s
(Fig.6).
3.0
bond strength [N/mm²]
diiodomethane (HD601CF)
100
contact angle [°]
water contact angle [°]
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
HD601CF
+0.2 % ADK 2112
+0.3 % Crodamide
2.5
2.0
1.5
1.0
0.5
0.0
untreated
4 ms (oxygen)
0.5 s (oxygen)
2 s (oxygen)
Fig.6 Bond strength of differently treated samples.
Compared to the untreated samples (HD601CF:
σ = (1.06 ±0.25) N/mm²) the bond strength more than
doubles
due
to
the
treatment
(0.5 s:
σ = (2.31 ±0.52) N/mm²). It is presumable, that a weak
boundary layer is starting to form on the surface of the
reference samples between treatment times of 0.5 s and
2 s. The same behavior can be seen for samples
containing additional anti-oxidants. The difference in
surface properties already identified in contact angle and
XPS measurements can be confirmed. Crodamide
samples show overall a weaker adhesion to the applied
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
SiOCH coatings. Furthermore, bond strength is already
declining after 0.5 s treatment time. The amides on the
surface weaken the interface of the system. A treatment in
oxygen plasma for a short time (< 0.5 s) is helpful
concerning coating adhesion but does not lead to a
positive effect for longer treatment times.
XPS measurements show a steep rise in functional
groups after only one pulse of oxygen plasma (Fig.7). For
longer treatments, a shoulder with a higher chemical shift
arises, suggesting the integration of oxygen into the
molecules.
1.2
no treatment
6. References
[1]
[2]
[3]
[4]
30 s (oxygen)
0.8
0.6
0.4
intensity [-]
1.0
1 pulse (oxygen)
[5]
0.2
0.0
292
290
288
286
bindung energy [eV]
284
282
[6]
Fig.7 C1s peak of untreated and in oxygen plasma treated
samples (+0.5 % Chimasorb).
[7]
4. Conclusion
The influence of additives onto the surface properties
and possible coating adhesion after a pre-treatment
strongly depends on the nature of the additives. The
investigated lubricant (Crodamide ER) has a negative
effect on coating adhesion. It migrates to the surface
forming a film which results in a weak boundary layer
after coating. The phenomena at the interface will be
addressed in further studies.
[8]
[9]
Other additives like the investigated anti-oxidant (ADK
Stab 2112) do not show a change in wetting behaviour or
coating adhesion in the frame of these investigations.
For PP a short treatment in a pulsed argon plasma is
suggested. To generate the optimal adhesion to a CVDcoating the composition of the material is of importance.
A precise control of plasma properties and treatment time
are necessary to ensure reproducibility. If there is a
change in base material, additives or the plasma
properties, the pre-treatment should be adjusted.
5. Acknowledgements
The depicted research has been funded by the
Deutschen Forschungsgemeinschaft (DFG / German
Research Foundation) as part of the Collaborative
Research Centre SFB-TR 87. We would like to extend our
thanks to the DFG. Furthermore we want to thank
Borealis Polyolefine GmbH for material support.
[10]
[11]
W. Michaeli, A. Hegenbart, D. Binkowski, F. v.
Fragstein: Steigerung der Klebfestigkeit von
PTFE und PFA durch Plasmabehandlung.
Zeitschrift Kunststofftechnik 3 (2007) 2, pp. 1-12
R. H. Hansen, H. Schonhorn: A new technique
for preparing low surface energy polymers for
adhesive bonding. Polymer Letters 4 (1966) 203209
M. Kuhr, S. Bauer, U. Rothhaar, D. Wolff:
Coatings on plastics with the PICVD technology.
Thin Solid Films 442 (2003) 1, pp. 107-116
A. Grill: Cold Plasmas in Materials Technology,
Institute of Electrical and Electronics Engineers,
New York, 1994.
G. Czeremuzskin, M. Latreche, M. R.
Wertheimer, D. Silva Sobrinho, A. S.: Ultrathin
Silicon-Compound
Barrier
Coatings
for
Polymeric Packaging Materials: An Industrial
Perspective. Plasmas Polym 6 (2001) 1, pp. 107120
H. Potente, R. Krüger: Oberflächenspannungen
und Haftfestigkeiten bei Corona-vorbehandeltem
PP. Adhäsion 12 (1979) 381-389
G. Rochat, Y. Leterrier, P. Fayet, J. E. Manson:
Influence of substrate additives on the
mechanical properties of ultrathin oxide coatings
on poly(ethylene terephthalate). Surface and
Coatings Technology 200 (2005), pp. 2236-2242
W. Petasch, E. Räuchle, H. Muegge, K. Muegge:
Duo-Plasmaline - A linearly extended
homogeneous low pressure plasma source.
Surface and Coatings Technology (1997) 93, pp.
112-118
C. Scharwitz, M. Boeke, J. Winter: Optimised
Plasma Absorption Probe for the Electron
Density Determination in Reactive Plasmas.
Plasma Process Polym 6 (2009) 1, pp. 76-85
B. Denis, S. Steves, E. Semmler, N. Bibinov, W.
Novak, P. Awakowicz: Plasma Sterilization of
Pharmaceutical Products: From Basics to
Production. Plasma Process Polym 9 (2012) 6,
pp. 619-629
M. Deilmann: Silicon oxide permeation barrier
coating and sterilization of PET bottles by pulsed
low-pressure microwave plasmas. RuhrUniversität Bochum, Dissertation, 2008