ADHESION OF SILICONE COATINGS TO
POLYPROPYLENE FILMS
50
Lesley-Ann O’Hare, Stuart R. Leadley, Bhukan Parbhoo, John G. Francis
Introduction.
The aim of this work was to elucidate the mechanisms of adhesion of a
silicone coating to a corona discharge treated polypropylene (PP) film. A
physico-chemical study of the PP surface utilising contact angle
measurements, X-ray Photoelectron Spectroscopy (XPS) and Atomic Force
Microscopy (AFM) was carried out. These techniques were used to probe
changes in the surface energy, surface chemistry and surface morphology, and
the results were related to practical adhesion data. Surface energies
investigated the role of physisorption, whilst chemisorption was examined by
the surface chemistry changes. The presence of mechanical interlocking was
studied by changes in the morphology, and a combination of these techniques
on washed samples scrutinised the presence of a weak boundary layer.
Experimental
Materials. The film used in this study was 30 µm thick, biaxially
oriented polypropylene (BOPP). The film was previously untreated, with no
surface contamination observed by X-Ray Photoelectron Spectroscopy.
The contact liquids used were water (HPLC Grade – Fisher); formamide
(99.5+% - Acros); diiodomethane (>98% - Fluka); ethane-1,2-diol (>99.9% Acros). The silicone coating system used reacted via addition cure chemistry,
comprising a vinyl- and a hydride-functional silicone polymer.
Equipment. Corona discharge treatment (CDT) was carried out on a
GX10 corona treater, manufactured by Sherman Treaters, Thame, Oxon. The
films were exposed to one pass under an electrode of width 0.4m, in ambient
air at a speed of 10 mmin-1. The various power settings used were converted
to energy values.
Contact angles of sessile drops were measured using an Advanced
Surface Technology video contact angle VCA2500 system. Contact angles
were measured on the modified side of the PP films within twenty minutes of
treatment. The surface energy results presented are the average of three
experiments carried out on consecutive days. The method of calculation used
in this study was the geometric mean approach of Owens-Wendt1 and Kaeble2.
X-ray Photoelectron Spectroscopy was performed using a Kratos
Analytical Axis Ultra instrument. A monochromated Al Ka X-ray source was
used at a nominal power of 300 W to record spectra at normal emission. All
of the samples under consideration required charge compensation.
Atomic Force Microscopy was carried out on a Digital Instruments
Dimension 3100, in the TappingMode® using a silicon tip. Both height and
phase images of scanned area 1µm2, 25µm2 and 400µm2 were captured.
Surface Energy (mJm-2)
40
Dow Corning Ltd., Cardiff Road, Barry, Vale of Glamorgan, CF63 2YL, UK
30
-2
Polar Contribution (mJm )
-2
Dispersive Contribution (mJm )
-2
Total Surface Energy (mJm )
20
10
0
0
2
4
6
8
10
12
14
Corona Energy (kJm-2)
Figure 1. Surface Energy (γ) and its polar (γp) and dispersive (γd) components
of corona discharge treated polypropylene film as a function of corona energy
As would be expected for a polypropylene film, the survey spectrum
showed carbon to be the main element present at the surface of the untreated
film. Survey spectra acquired from the corona treated films showed carbon
and oxygen to be the only elements present at the surface. This indicates that
the polar groups introduced by CDT contain oxygen. The amount of oxygen
incorporated typically increased with increasing corona energy. Between 5
and 11% atomic oxygen is introduced to the surface over the energy range
studied.
By fitting peaks to the high-resolution C 1s spectra, it was also possible to
identify the specific type of functional groups introduced at the surface by
CDT. Corona treatment introduces a shoulder on the high binding energy of
the C 1s spectrum, Figure 2.
a
Results and Discussion
Physical Adsorption Theory. Surface energy (γ) measurements
interrogate the polar (γp) and dispersive (γd) components of a substrate. The
role of the physical adsorption theory in adhesion of silicone to BOPP film
can thus be investigated. Figure 1 shows the values obtained for γ, γp and γd
plotted as a function of the energy delivered to the surface of the
polypropylene film by corona discharge.
Before CDT, the surface energy of the polypropylene film is composed
solely of a dispersion component. As expected, γ and γp increase with
increasing energy of CDT. However, the shape of the curve for γp is not the
same as that of γ. This indicates that at certain CDT energy levels (2.25 to
4.05 kJm-2), the surface energy does not correlate with the increase in polarity
alone. Since the dispersive component remains relatively constant across the
energy range studied, including the untreated film, enhanced adhesion after
CDT is not caused by the physisorption mechanism.
Chemical Adsorption Theory. The chemisorption theory was probed
by monitoring the changes in the surface chemistry of the BOPP due to CDT.
The introduction of any functional groups to the surface of the BOPP that
could interact with the silicone coating would imply that chemical adsorption
was an important mechanism of adhesion.
b
Figure 2. High resolution spectra of polypropylene film
a) untreated film, b) treated film, corona energy = 15 kJm-2
In agreement with studies published elsewhere3,4,5, this work has assigned the
functional groups introduced by CDT of BOPP as hydroxyl, carbonyl, peroxy,
ester, carboxylic acid and anhydride groups. It was observed that the number
of peaks that could be fitted, and their relative areas, varied depending on the
energy of corona discharge. At lower energy levels it was not possible to fit
all six additional peaks. It is proposed, therefore, that the type of functional
groups introduced to the surface of polypropylene films, and their relative
concentrations, are dependant on the energy delivered to the surface by CDT.
The changes in the relative concentrations of the species introduced are
presented as a function of the energy of corona in Figure 3.
Polymer Preprints 2001, 42(1), xxxx
Table 1. Roughness values for a series of corona treated polypropylene films.
Corona Energy (kJm-2)
RRMS (nm)
Relative Concentration (%)
3.5
3
C-OH
2.5
C-O-O
C=O
2
C-O-C*=O
1.5
HOC=O
C(O)OC(O)
1
0.5
0
0
5
10
15
Corona Energy (kJ/m2)
Figure 3. The functionalities introduced by Corona Discharge Treatment as a
function of corona energy
Hydroxyl, peroxy and carbonyl functional groups are introduced at all levels
of CDT. The more highly oxidised species, however, are not introduced until
more aggressive energy levels have been employed. More surprisingly, these
carboxylic acid, ester and anhydride groups increase in concentration at
energy levels where the atomic concentration of oxygen is actually decreasing
slightly. This suggests that until the energy level of ~ 4 kJm-2 is crossed, the
main mechanism of CDT is oxidation and crosslinking; above this energy
level, a chain scission mechanism is also occurring.
The chemisorption mechanism of enhanced adhesion must be of
importance; the nature of such functional groups as discussed previously may
affect the performance of any particular adhesive system. This is likely to be
true for this type of reaction; the highly reactive Si-H species in the adhesive
may react with functional groups on the surface. This supposition has yet to
be confirmed, and further work will be necessary to determine any rates of
reaction that may occur. These results must also be compared in turn with the
rate of hydrosilylation of the vinyl functional groups in the bulk silicone
Mechanical Interlocking. Increased practical adhesion after CDT has
often been attributed to mechanical interlocking due to increased roughness.
AFM was used to monitor any morphological changes induced by CDT, and
then to relate these to any variation in the adhesive properties of
polypropylene film. The biaxial orientation of the untreated film, as observed
by Boyd et al6 was apparent in the 1µm2 image. The defined fibrillar structure
often commented on for untreated PP was observed, Figure 4a. After CDT,
however, a different morphology emerges; globular features of 50 –100 nm
diameter are observed, Figure 4b. At energy levels between 0 and 5.7 kJm-2,
both the fibrillar and globular morphologies can be observed, due to the
heterogeneity of the treatment at low energy levels. Figure 4c shows again a
different morphology. In this image, a branchlike structure is observed. It
may be surmised that at this high energy level the treatment is exposing the
underlying polymer structure. The low molecular weight boundary layer
common to polyolefins, and perhaps parts of the amorphous regions, has been
ablated under these conditions.
0
2.4
3.15
2.2
5.70
2.6
15.0
4.2
To try to avoid ambiguities regarding the fractal nature of the surface, and any
changes in roughness that may incur, all roughness values were calculated on
a 1 µm2 area. It is clear that under these conditions no significant increase in
roughness occurs. Previous studies that reported an increase in roughness by
CDT utilised longer treatment times and higher powers. This suggests that in
the case of silicone adhering to polypropylene film, mechanical interlocking is
not a dominant mechanism of adhesion.
Weak Boundary Layer. The formation of a layer of low molecular
weight oxidised material (LMWOM) on the surface of PP film after CDT is
widely accepted7. It is the effect of the material on adhesion that is debated.
The formation of a weak boundary layer may be beneficial or detrimental to
adhesion, dependant on its solubility in the adhesive matrix. If the layer is
soluble in the matrix, no decrease in adhesion may be observed. If, however,
the layer is not soluble, this may reduce adhesion due to the presence of this
cohesively weak layer at the interface. Water washing experiments were
carried out to evaluate the effect of LMWOM on the surface chemistry and
morphology. XPS and AFM analyses were carried out under the same
conditions as used for the unwashed samples. XPS identified that around 50%
of the oxidised material on the surface is in the form of water-soluble
LWMOM. Since after washing the atomic oxygen content does not decrease
to that of the untreated film, this indicates that some of the oxygen
incorporated is firmly bound to the polymer backbone.
AFM also reinforces the theory that not all the material is removed by
washing, since although the globular morphology observed in Figure 4b is
removed, the structure does not return to that of the untreated film, Figure 4a.
The amount of LMWOM removed by water washing varies with the energy
imparted to the surface of the film. The greatest change is observed on the
film treated at the highest corona energy. It is possible that the presence of
this material may form cohesively weak layer at the interface that is insoluble
in the silicone adhesive matrix. Thus weak boundary layer is an important
mechanism in the adhesion of silicone to CDT polypropylene.
Practical Adhesion Measurements. The anchorage of the silicone
coating to the film has been evaluated using the anchorage index test. The test
was carried out on films treated at all CDT energy levels, with siliconising
being carried out of films both with and without water washing. The results
clearly showed that removal of LMWOM by water washing prior to
siliconising is beneficial to adhesion over a 1 month time period, particularly
at the highest treatment energy.
Conclusions
The effect of corona discharge treatment on the surface physicochemistry of biaxially oriented polypropylene has been investigated using
surface energy measurements from contact angles, X-ray Photoelectron
Spectroscopy and Atomic Force Microscopy. The information gathered from
these techniques, in addition to practical adhesion measurements, has enabled
identification of the dominant mechanisms of adhesion of silicones to PP
films. The physisorption mechanism and the mechanism of mechanical
interlocking are not the cause of enhanced adhesion after corona discharge
treatment in this system. It is believed that the dominant mechanisms of
adhesion of silicone to corona treated polypropylene are chemisorption and
weak boundary layer.
References
1
a
b
c
Figure 4: AFM images of polypropylene film
a) 1 x 1µm untreated, b) 1 x 1 µm treated film, corona energy = 5.70 kJm-2,
c) 5 x 5 µm treated film, corona energy =15.0 kJm-2
In addition to observing the changes in morphology, the root mean square
roughness (RRMS) was also calculated, Table 1.
Owens, D.K.; Wendt, R.C. J. Appl. Polym. Sci. 1969, 13, 1741
Kaeble, D.H. J.Adhesion 1970, 2, 66
3
Foersch, R.; McIntyre, N.S., J. Polym. Sci.: Part A: Polym. Chem. 1990, 28,
193
4
Comyn, J., Adhesion Science, Ch.1, RSC Paperbacks
5
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17(2), 156
6
Boyd, R.D.; Kenright, A.M.; Badyal, J.P.S., Macromolecules 1997, 30, 5429
7
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M.C., J. Adhesion Sci. Technol, 1989, 3(5), 321-335,
2
Polymer Preprints 2001, 42(1), xxxx