Advances in Materials Physics and Chemistry, 2011, 1, 50-55
doi:10.4236/ampc.2011.12009 Published Online September 2011 (
Copyright © 2011 SciRes. AMPC
Plasma Surface Functionalization of Biaxially
Oriented Polypropylene Films with Trimethyl Borate
Nursel Dilsiz1*, Hande Yavuz1, Süleyman Çörekçi2, Mehmet Çakmak3
1Department of Chemical Engineering, Gazi University, Ankara, Türkiye
2Department of Physics, Kırklareli University, Kırklareli, Turkey
3Department of Physics, Gazi University, Ankara, Türkiye
E-mail: *
Received May 1, 2011; revised June 19, 2011; accepted June 18, 2011
The radiofrequency plasma (13.56 MHz) was employed to polymerize trimethyl borate (TMB) monomer/N2
gas mixture on the surface of biaxially oriented polypropylene (BOPP) films. Plasma polymer coated poly-
propylene films were examined by flame retardancy test (limiting oxygen index, LOI). The highest LOI
value calculated for the untreated BOPP sample was 18.4 (v/v O2%) and 24.2 (v/v O2%) for the 55 W 30
minutes treated sample. The plasma polymers were characterized by FTIR spectroscopy and AFM. Accord-
ing to the FTIR results, the -OH, B-CH3, B-O, and BH2 functional groups were detected. It was found that
the highest surface roughness belonged to 40 W 30 min treated BOPP sample which was calculated as 9.78
nm (10 μm × 10 μm). Moreover, the wettability of the modified BOPP film surfaces was characterized via
contact angle measurements. The water contact angle values have decreased from 109.6˚ to the lowest value
of 68.2˚ after the plasma treatment. The results showed that TMB/N2 plasma modification could be used as
an alternative method for the enhancement of flame retardancy and hydrophilicity of BOPP film.
Keywords: Polypropylene Film, Trimethyl Borate, Plasma Treatment, Flame Retardancy, Hydrophilicity
1. Introduction
It is well known that the thermal and the mechanical pro-
perties of the polyolefins vary with their type and the
degree of their crystallinity. Some of these polymers su-
ch as low modulus and low tenacity type polypropylene
exhibit low thermal resistance and low tensile strength.
In order to enhance their thermal resistivity and mech-
anical strength, surface modification or layer deposition
in micrometer or nanometer scale may be employed.
Among all other modification processes plasma modi-
fication draws great attention over the past years due to
the discovery of its various advantages comparing to
conventional methods [1]. Cold plasma process, which is
an environmentally friendly and safe method, allows the
surface of any polymer to be changed to achieve the de-
sired functionality while retaining the bulk properties of
material. Due to the fragmentation of organic comp-
ounds one may not always obtain the expected polymer
formations by plasma polymerization [2]. In other words,
organic vapor polymerization under the influence of pla-
sma is quite complex and can not be specifically des-
cribed by general cases. In plasma treatments, a wide
variety of monomers can be used for various applications.
According to the applied plasma conditions and selected
substrate types, desired functional film layer formations
can be obtained in a very short time. For example, mon-
omers such as oxygen based ones can lead to the forma-
tion of carboxylic acids, ketones and esters where, nitro-
gen based monomers show high level of nitrogen incur-
poration into the deposited polymer layer [3]. Typically,
these plasma polymers have high degree of branching
and crosslinking in comparing to conventional polymeric
structures [2]. Jama et al. [4-6] reported first promising
results for improvement of fire behavior of polymer sur-
face by plasma modification with organosiloxanes. Sch-
artel et al. [7] described the surface controlled fire retar-
dancy of polyamide films using plasma polymerization
of hexamethyldisiloxane. According to the authors, plas-
ma polymer growth on the suitable substrate surfaces can
be optimized by controlled plasma process parameters
and the resultant chemical structure of the plasma coated
layer could be sufficient to prevent the entire combustion
of material. Besides, many types of flame retardants
which are available in consumer products are mainly
composed of phosphorus, antimony, aluminum, chlorides,
bromides [8-10] and boron-containing compounds. Dur-
ing the thermal degradation of polymers, borates leads to
the formation of impenetrable glass coatings on the sur-
face of materials, therefore they can be classified in the
effective flame retardants [11].
In this current research, BOPP films were modified in
TMB/N2 gas mixture in order to improve the flame re-
tardancy of polymer films. To describe and to measure
the properties of BOPP films response to heat and flame,
Limiting Oxygen Index (LOI) method was evaluated.
Fourier Transform Infrared Spectroscopy (FTIR) was
used in the determination of the chemical structure of
functionalized surface. Atomic Force Microscopy (AFM)
and wettability measurements were carried out to char-
acterize the surface properties of BOPP films. The results
showed that TMB/N2 plasma modification could be used
as an alternative method for the enhancement of flame
retardancy and hydrophilicity of BOPP film.
2. Experimental
2.1. Materials
Biaxially oriented polypropylene films (BOPP, thickness:
40 μm), which were supplied by Super Film A.S. (Ga-
ziantep, Turkey), were used as received. Trimethyl borate
(TMB), B(OCH3)3 was supplied by Fluka (Germany). The
code of materials used in this study is given in the Table 1.
2.2. Surface Modification of BOPP Films
The radio frequency (RF) plasma was generated by
Diener Electronic Pico UHP system (Germany) at 13.56
MHz with an electrical power ranging between 0 W and
100 W. The applied discharge power of 30 W, 40 W, 55
W, and 80 W was chosen. The impedance match be-
tween the reactor chamber and the radiofrequency gen-
erator was optimized by the matching network, which
provided power loses at the minimum. The dimension of
a cylindrical shaped quartz plasma reactor was 130 mm
(diameter) × 300 mm (length) and two copper ele-
ctrodes were located on the reactor externally. A combina-
tion of cold trap and vacuum pump was used to maintain
the pressure range in the reactor between 0.1 mbar and 1
mbar. In the absence of the TMB/N2, the pressure of the
reactor was decreased to about 0.1 mbar. The BOPP film
and NaCl crystal were used as the substrates onto which
the plasma polymer films were deposited and were placed
in the reactor and treated simultaneously with the same
plasma conditions. The plasma polymer coated on the NaCl
crystals were used to determine the FTIR spectra of the
Table 1. Code of materials used in this study.
Specimen code Description
BOPP Untreated biaxially oriented polypropylene
BOPP-N2TMB30W30M At 30 W and for 30 min nitrogen gas and
trimethyl borate plasma treated BOPP film
BOPP-N2TMB40W30M At 40 W and for 30 min nitrogen gas and
trimethyl borate plasma treated BOPP film
BOPP-N2TMB55W30M At 55 W and for 30 min nitrogen gas and
trimethyl borate plasma treated BOPP film
BOPP-N2TMB80W30M At 80 W and for 30 min nitrogen gas and
trimethyl borate plasma treated BOPP film
NaCl Untreated pure NaCl crystal
NaCl-N2TMB30W30M At 30 W and for 30 min nitrogen gas and
trimethyl borate plasma treated NaCl crystal
NaCl-N2TMB40W30M At 40 W and for 30 min nitrogen gas and
trimethyl borate plasma treated NaCl crystal
NaCl-N2TMB55W30M At 55 W and for 30 min nitrogen gas and
trimethyl borate plasma treated NaCl crystal
NaCl-N2TMB80W30M At 80 W and for 30 min nitrogen gas and
trimethyl borate plasma treated NaCl crystal
polymer film. The substrates were pretreated in the
plasma chamber with argon gas before each plasma op-
eration to clean and to improve the adhesion of the flame
retardant coating. When the TMB/N2 mixture was fed
into the reactor by a needle valve, the pressure was ad-
justed to 0.26 mbar - 0.30 mbar and was kept at that level
during the plasma operation. Polymerization were carried
out for TMB/N2 under different discharge power and
plasma exposure time was kept constant at 30 minutes.
Additionally, the effluents from the reactor in which
plasma polymerization took place were discharged th-
rough a piping system. After the completion of treatment,
argon was fed into the reactor for 15 min to deactivate
free radicals. For a given conditions, all experiments
were carried out as triplicate.
2.3. Characterization of Plasma Surface
Modified BOPP Films
FTIR spectra of the plasma polymers deposited on NaCl
were recorded by using Jasco 480 Plus FTIR spectrome-
ter having an 8 cm–1 resolution. FTIR spectroscopy was
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Copyright © 2011 SciRes. AMPC
used to determine the functional groups present in a
molecule. The infrared region was taken from 4000 cm–1
to 400 cm–1 wavenumbers where the most vibrations
mainly occur.
med on Krüss100 DSA equipped with a fast CCD camera.
Sessile drop type was selected for the static contact angle
setup, and distilled water was used for the each contact
angle measurement. Five separate TMB/N2 plasma modi-
fied films were measured to get an average value.
In order to determine the effect of the deposited film
layer on the surface topography, the BOPP samples were
examined by atomic force microscopy (AFM). Surface
topography of TMB/N2 plasma modified BOPP film was
investigated by AFM operated in tapping mode on an
Omicron VT STM/AFM Instrument. The main objective
of the collection of AFM images is to measure the ato-
mic level forces between the sample surface and the pro-
be. Acquiring images over a small area is possible with
the small probe-specimen separation, thus, in this work
scanning ranges were adjusted to 10 μm × 10 μm. The
images were collected while the tip was operated at Tap-
ping Mode. Roughness (RMS) was determined as a mean
from five AFM scans over the same area.
3. Results and Discussion
3.1. FTIR Spectroscopy
The chemical structure of plasma modified BOPP film,
which was treated under N2 gas and TMB vapor, indicated
that at the surface mainly boron based functional groups
were included. Since the methylene deformations mainly
occur between 1500 cm–1 and 1300 cm–1 wave numbers,
plasma polymer coated NaCl crystals were used to help the
identification of B-CH3, B-O, BH2 functional groups which
consistently produce absorption bands between 1460 cm–1 -
1405 cm–1, 1350 cm–1 - 1310 cm–1 and 1205 cm–1 - 1140
cm–1, respectively. In order to relate plasma power effect
with constant plasma exposure time (at 30 min), the FTIR
spectra were collected as represented in Figure 1. It can be
seen that almost all of the plasma polymers obtained from
different (30 W, 40 W, 55 W and 80 W) discharge power
gave similar characteristic peaks, indicating similar func-
tional groups in the plasma polymers. In general the relative
intensities of these peaks have varied with increasing dis-
charge power. One of the major peaks observed for all
plasma polymers was obtained at 3450 cm–1 - 3100 cm–1,
which is mostly due to O-H stretching band. The peak at
1460 cm–1 - 1405 cm–1 can be assigned to B-CH3
symmetrical deformation and absorption at 1205 cm–1 -
1140 cm–1 may be assigned to BH2 in-plane deformation
[14]. Aditionally, the formation of B-O stretching band was
observed at 1350 cm–1 - 1310 cm–1 at 30 W discharge power.
Fire response of plasma modified BOPP films were
studied according to the LOI test method under contro-
lled laboratory conditions described in ASTM D 2863-00
standard [13]. Dynisco LOI polymer test equipment was
used to determine the flammability of the films according
to the ASTM D 2863 standard. According to the test
method, flexible films (e.g. BOPP films) should be sup-
ported vertically in the center of a heat resistant transpar-
ent chimney and tested in accordance with the test method
B. At least fifteen specimens were prepared in order to
maintain LOI and fire response unmodified and each
plasma modified sample. In order to assess fire response
of individual test specimens, period and extent of burning
were considered. Dixon’s “Up-and-Down” method, which
is described in ASTM D 2863-00 standard, was used to
calculate the LOI of each series of specimens.
The wettability tests for the BOPP films were perfor-
Figure 1. FTIR spectra of plasma polymers at exact plasma exposure time a) 30 W, b) 40 W, c) 55 W, d) 80 W.
The formation of new groups in the TMB plasma
polymer (BH2, B-CH3, etc.) is mainly caused by the
fragmentation of monomers under plasma conditions.
Due to fragmentation of organic compounds in plasma
the polymer formed is not always what one would expect
from the chemical structure of the monomer [15]. When
the discharge power was increased to 55 W, the inten-
sities of the peaks has increased slightly. The absorption
intensities for all plasma polymers seemed to increase
first upon increasing the discharge power up to 55 W.
When the discharge power was raised to 80 W the
intensities of all the peaks have decreased. This result
can be explained by the fact that by increasing discharge
power a growing portion of the modified polymer is
ablated [16] and thinner layer of modified polymer is
available for subsequent FTIR measurement. It can be
concluded that the decrease in the peak intensites may be
due to increased decomposition of the deposited poly-
mers with increasing power.
3.2. Limiting Oxygen Index (LOI)
Results of oxygen index test are given in Table 2. The
results demonstrate that TMB/N2 plasma polymers modi-
fied BOPP film have higher LOI value than the untreated
BOPP film. From Table 2 it can be seen that when
plasma power increases to 30 W, LOI value TMB/N2 pl-
asma treated BOPP film increases from 18.4 (untreated
BOPP) to 23.6 (%O2, v/v). When the plasma power in-
creases to 55 W, the maximum LOI value obtained was
24.2 (%O2, v/v). The LOI difference between untreated
and plasma modified films was calculated as approxi-
mately 30%. This trend could be related to the deposition
of B-O functional groups rather than the deposition of
B-C and/or B-H functional groups, which can be formed
in the cold plasma environment, on the surface of the
BOPP films. This conclusion was also supported by
FTIR spectra given in Figure 1.
3.3. Atomic Force Microscopy
The effect of plasma discharge power on the surface
morphology of TMB/N2 modified BOPP can be seen
Table 2. LOI data for plasma treated BOPP film.
Specimen code LOI (O2%, v/v)
Untreated BOPP 18.4 ± 0.20
BOPP-N2TMB30W30M 23.6 ± 0.18
BOPP-N2TMB40W30M 22.5 ± 0.16
BOPP-N2TMB55W30M 24.2 ± 0.27
BOPP-N2TMB80W30M 22.4 ± 0.21
from AFM images shown in Figure 2. As seen from the
images, surface morphologies of the treated samples
slightly changed compared to the untreated one.
Roughness was determined as a mean from five AFM
scans over the same area and its dependence on dis-
charge power is given in Table 3. As is shown in Figure
2, the effects of different power on surface morphologies
of BOPP film were characterized by AFM. For the
untreated BOPP film, the surface roughness was 5.64 nm.
However, after 40 W TMB/N2 plasma treated for 30 min,
the fiber surface roughness increased to 9.78 nm. It was
found that the untreated BOPP film had a smooth surface
(Figure 2(a)). It can be found that BOPP films were
notably roughened by the plasma power. The surface
roughness of plasma polymer modified BOPP films
enhanced with the plasma power up to 40 W then the
surface rougness slightly decrease with increasing the
plasma treatment power. These results indicate that
BOPP film were notably roughened by the TMB/N2
plasma treatment.
3.4. Contact Angle
Under various discharge conditions, it is found that the
chemical structure of the top BOPP film layer was
changed significantly. The relation between the treatment
conditions and contact angle values can be followed in
Table 4. It is known that plasma polymer deposition
Table 3. Surface roughness of untreated and plasma po-
lymerized BOPP film.
Surface roughness (nm)
Sample code 10 µm × 10 µm
BOPP 5.64 ± 0.83
BOPP-N2TMB30W30M 7.27 ± 0.72
BOPP-N2TMB40W30M 9.78 ± 1.25
BOPP-N2TMB55W30M 7.85 ± 0.52
BOPP-N2TMB80W30M 6.22 ± 0.48
Table 4. Contact angle values of untreated and plasma po-
lymerized BOPP film.
Specimen code Contact Angle (°)
Untreated BOPP 109.6 ± 0.25
BOPP-N2TMB30W30M 73.6 ± 2.65
BOPP-N2TMB40W30M 68.2 ± 1.58
BOPP-N2TMB55W30M 88.6 ± 5.00
BOPP-N2TMB80W30M 83.3 ± 0.72
Copyright © 2011 SciRes. AMPC
Figure 2. AFM images of a) Untreated BOPP film; b) BOPP-N2TMB30W30M; c) BOPP-N2TMB40W30M; d) BOPP-N2
TMB55W30M; e) BOPP-N2TMB80W30M (10 μm × 10 μm).
depends strongly on the nature of monomer type and
discharge power. TMB/N2 plasma led to the formation of
polar groups (e.g. B-O, -OH) on the surface and caused
the removal of some C-H groups from the surface of
BOPP film. So that, wettability of surface was increased,
samples became more hydrophilic comparing to the
original value (108.3˚). While the highest contact angle
was obtained at 55 W (88.6˚), the lowest one was ob-
tained at 40 W (68.1˚). This can be attributed to the in-
troduction of hydrophilic groups (-OH) on the polymer
surface which was seen on the FTIR spectra. All these
results are quite interesting from the point of view of
seeing no linear trend between the discharge power and
contact angle. The differences among these values can be
attributed to the change in distribution of polymer depo-
sition due to the applied conditions. There seems to be no
obvious correlation between surface roughness and dis-
charge power. It can be seen that AFM images of
TMB/N2 plasma polymers and their structure have
changed considerably upon increasing discharge power.
These results follow the same tendency as reported by
Svorick et al. [16] It is known that the value of the con-
tact angle is mostly affected by the chemical structure
and morphology of the polymer surface layer [17].
Various reactions that takes place during plasma poly-
merization include excitation, ionization, homolitic bond
splitting, molecular fragmentation etc. For a low level of
energy input, the extent of ionization is low and ion-
molecule reactions dominate polymer formation resulting
in deposition rate increase with power [18]. Above
certain levels of energy, there is greater loss of functional
groups from the monomer resulting in compe- titive
polymerization and ablation [19].
4. Conclusions
We have focused on the plasma modification of BOPP
films in the presence of TMB/N2 in order to improve
their tolerance to flame. Thus, in this work biaxially ori-
ented polypropylene films have been examined mainly
by means of their LOI values. It has been stated that the
LOI values can be optimized by adjusting the discharge
power and time. When the treatment time kept constant
at 30 minutes, and the plasma power was adjusted to 55
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W, LOI of BOPP films obtained have increased from
18.4 to 24.2. It can be seen that the plasma modified
BOPP films have gained resistance to flame comparing
to unmodified samples. Besides, the effects of plasma
polymer deposition on the surface of BOPP film samples
were studied from the AFM and FTIR results. These
results have showed that boron-based functional layer
deposition can be applied to enhance the flame retar-
dancy of BOPP films.
5. Acknowledgements
This work was supported by National Boron Research
Institute (BOREN) contact grant number BOREN-2006-
Ç-02 and The State Planning Organization (DPT) contact
grant number 2001K-120590.
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