Engineering, 2010, 2, 397-402
doi:10.4236/eng.2010.26052 Published Online June 2010 (
Copyright © 2010 SciRes. ENG
Role of Plasma Surface Treatments on Wetting and
Rory Wolf1, Amelia Carolina Sparavigna2
1Enercon Industries Corporation, Menomonee Falls, Wisconsin, USA
2Dipartimento di Fisica, Politencico di Torino, Torino, Italy
Received December 9, 2009; revised February 13, 2010; accepted February 18, 2010
There are many current and emerging wetting and adhesion issues which require an additional surface proc-
essing to enhance interfacial surface properties. Materials which are non-polar, such as polymers, have low
surface energy and therefore typically require surface treatment to promote wetting of inks and coating. One
way of increasing surface energy and reactivity is to bombard a polymer surface with atmospheric plasma.
When the ionized gas is discharged on the polymer, effects of ablation, crosslinking and activation are pro-
duced on its surface. In this paper we will analyse the role of plasma and its use in increasing the surface en-
ergy to achieve wettability and improve adhesion of polymeric surfaces.
Keywords: Atmospheric Plasma, Surface Treatments, Polymeric Films
1. Introduction
The packaging industry is experiencing a technological
revolution aimed at increasing consumer convenience
and protection, and delivering new solutions for manu-
facturing and the distribution chain. Many wetting and
adhesion issues are therefore emerging, which require an
additional surface processing to promote wetting of inks
and coating and enhance adhesion with these and other
substances, to obtain high performance composited struc-
tures with polymeric and metallic foils.
Adhesion is a manifestation of attractive forces among
atoms. There is a general agreement on the fact that at-
tractions due to hydrogen and Van der Walls bonding are
sufficient to produce an adhesive joint between polymers
[1]. When adhesion of polymers with other materials
such as metallic foils is necessary, chemical reactions at
the interface leading to covalent chemical bonds are re-
quired [1,2]. This means that it is necessary the presence
of highly reactive functional groups at the surface.
One way of increasing surface energy and reactivity is
to treat the surface of the polymeric substrate with at-
mospheric plasma, which is an ionized gas at atmos-
pheric pressure. When plasma gas is discharged on the
polymer, effects of ablation, crosslinking and activation
are produced on its surface.
While corona, flame and priming surface pre-treat-
ments have been traditionally used in preparing finished
flexible packaging structures for graphic and coating
enhancements, the technology of atmospheric plasma
treaters is giving evidence of performance benefits in
increasing wettability and adhesion of polymeric sur-
faces [2-8]. More recently, experiments with plasma
treatment coupled with grafting copolymerization [9-11],
and used to enhance vapour depositions, show evidence
for clear barrier deposition. These barriers, according to
their relative deposition procedures, displayed hydro-
phobic, hydrophilic, anti-fog, biocide and anti-bacterial
results. These very recent plasma treatments can have
several applications for medical polymers [12,13].
The atmospheric plasma treatment (APT) process was
developed for treating/functionalizing a wide range of
materials and has advantages over the presently used
technologies of corona, flame, and priming treatments
for flexible packaging applications. The APT system
allows the creation of uniform and homogenous high-
density plasma at atmospheric pressure and at low tem-
perature, utilising a broad range of inert and reactive
gases [14,15]. Here we will discuss the role of APT in
increasing the energy of polymeric surface. The energy
produces a consequent increase in wettability and adhe-
2. Atmospheric Plasma Treatment Processes
If a substrate has a low surface energy, its wettability is
poor and coating adhesion very scarce, and then needs a
surface treatment to increase energy. There are several
Copyright © 2010 SciRes. ENG
methods for non-polar substrates. Those that can be con-
figured as in-line treatment systems serve as economical
alternatives to chemical primers, batch-treating processes,
speciality coatings, and adhesives. These in-line treat-
ments are mainly corona, flame and atmospheric plasma.
A flame system creates a flame plasma field when flam-
mable gas and air are combined and combusted to form a
blue flame [8]. Brief exposures to particles within the
flame affect the distribution and density of electrons on
the substrate and polarise surface molecules through
oxidation. This method also deposits other functional
chemical groups that further promote ink wetting and
A corona treating system is designed to increase the
surface energy of plastic films, foils and paper in order to
improve wettability and adhesion [16]. A corona treating
system consists of two major components: the power
supply and the treater station. The treater applies the
power to the substrate, through an air gap, via a pair of
electrodes, one at high potential and a roll supporting the
material at the ground potential. Only the side of the ma-
terial facing the high potential electrode should show an
increase in surface energy.
Much like a corona discharge, an atmospheric plasma
discharge is generated at atmospheric pressure. Instead
of using air, this method relies on other gases that deposit
specific chemical groups on the substrate surface to im-
prove its surface energy and adhesion characteristics.
APT process acts on material surfaces in a way which
is similar to the vacuum plasma treatment process. APT
production equipment testing has been performed for the
treatment of several materials, including the most utilised
in applications such as polypropylene, polyethylene, poly-
ester, polyamide, and polytetrafluoroethylene. The sur-
face energies of the treated materials increased substan-
tially, without any backside treatment or pin-holing [17],
thereby enhancing wettability, printability, and adhesion
The APT process consists of exposing a polymer to a
low-temperature, high density glow discharge. Figure 1
shows the air gap, the dimension of which can range
from 1.0 mm to 2.5 mm, between the electrodes occu-
pied by the glowing reacted gas. Polymeric films run
above the lower electrode connected to the ground.
The resulting plasma is a partially ionised gas consist-
ing of large concentrations of excited atomic, molecular,
ionic, and free-radical species. Excitation of the gas
molecules is accomplished by subjecting the gas, which
is delivered within an open station design, to an electric
field, typically at high frequency. Free electrons gain
energy from the imposed high frequency electric field,
colliding with neutral gas molecules and transferring
energy, dissociating the molecules to form numerous
reactive species. It is the interaction of these excited spe-
Figure 1. Air gap between the electrodes of Enercon plasma
treater, occupied by the glowing gas. Gap dimension can
range from 1 mm to 2.5 mm. Polymeric films run above the
lower electrode connected to the ground.
cies with solid surfaces placed in opposition to the
plasma that results in the chemical and physical modifi-
cation of the material surface.
The effect of plasma on a given material is determined
by the chemistry of the reactions between the surface and
the reactive species present in the plasma. At the low
exposure energies typically used for surface treatment,
the plasma surface interactions only change the surface
of the material; the effects are confined to a region only
several molecular layers deep and do not change the bulk
properties of the substrate.
The resulting surface changes depend on the composi-
tion of the surface and the gas used. Gases or mixtures of
gases, used for plasma treatment of polymers can include
N, Ar, O2, He, nitrous oxide, water vapour, carbon diox-
ide, methane, ammonia, and others. Each gas produces a
unique plasma composition and results in different sur-
face properties. For example, the surface energy can be
increased very quickly and effectively by plasma-in-
duced oxidation. Depending on the chemistry of the
polymer and the source gases, substitution of molecular
moieties into the surface can make polymers very wet-
table. The specific type of substituted atoms or groups
determines the specific surface potential.
3. Ablation, Crosslinking and Activation
For any gas composition, three surface processes simul-
taneously alter flexible packaging substrates, with the
extent of each depending on the chemistry and process
variables: ablation, crosslinking, and activation [18].
In the ablation process, the bombardment of the poly-
mer surface by free radicals, electrons, ions and radiation
breaks the covalent bonds of the polymer backbone, re-
sulting in lower-molecular-weight polymer chains. As
long molecular components become shorter, the volatile
oligomer and monomer by-products vaporise off (ablate)
and are swept away with exhaust.
Crosslinking is done with an inert process gas (Ar or
He). The bond breaking occurs on the polymer surface.
But since there are no free-radical scavengers, it can
form a bond with a nearby free radical on a different
chain (crosslink).
Activation is a process where surface polymer func-
tional groups are replaced with different atoms or che-
mical groups from the plasma. As with ablation, surface
exposure to energetic species abstracts hydrogen or
breaks the backbone of the polymer, creating free radi-
cals. In addition, plasma contains very high-energy UV
radiation. This UV energy creates additional free radicals
on the polymer surface. Free radicals, which are ther-
modynamically unstable, quickly react with the polymer
backbone itself or with other free-radical species present
at the surface to form stable covalently bonded atoms or
more complex groups.
Application of atmospheric plasma to finished films
has been theorised and practised to provide specific
functionality to the base film substrate adequate for im-
proved adhesion relative to the corona treatment process
[19,20]. Since atmospheric plasma contains highly reac-
tive species within the high density plasma at atmos-
pheric pressure, it is proven to significantly increase sur-
face area and to create polar groups on the surface of
polymers so that strong covalent bonding between the
substrate and its interface (i.e. , inks, coatings, and adhe-
sives) takes place.
It was therefore interesting to modify the polymeric
surface in atmospheric plasma and study any change,
relative to the untreated surface, of atomic bonding. This
surface characterization of untreated and treated poly-
meric films can be obtained using the Electron Spec-
troscopy for Chemical Analysis (ESCA). Surface analy-
sis by ESCA (also known as XPS, X-ray photoelectron
spectroscopy) is accomplished by irradiating a sample
with soft X-rays and analyzing the energy of ejected
electrons. When the sample is irradiated, the resulting
photoelectrons have energy depending on their original
binding energy. A typical ESCA spectrum is a plot of the
number of electrons detected per unit of time versus their
binding energy. Each element produces a characteristic
set of peaks at specific binding energy values, corre-
sponding to the electron configurations within atoms.
The number of detected electrons in each peak is directly
related to the amount of element within the area irradi-
ated. ESCA then could be used to identify and determine
the concentration of the elements on the surface [21].
Figure 2 shows the C1s ESCA spectra for an untreated
PET film and PET films after Ar/O2 plasma treatments.
The X-ray photoelectron spectroscopy analysis suggests a
modification of the surface when subjected to a plasma
treatment. In particular, comparing C1s spectra of un-
treated and treated samples, we note that number of func-
tional groups is clearly changed. Let us remember that
C1s spectrum is coming from three different groups of
carbon atoms: the relative intensities of the structures at
289, 286.5 and 285 eV corresponds to C atoms in OC=O,
CCO and CCC bonding positions.
Figure 2. C1s ESCA spectra as a function of binding ener-
gies for untreated PET film and PET films after corona and
Ar/O2 plasma treatments.
Figure 2 is also showing the spectrum of a corona
treated sample. The intensity of the highest peak, the aro-
matic peak, decreases after plasma treatment, whereas the
corona treatment seems to leave it unchanged. The rela-
tive intensity of second peak at 286.5 eV is reduced in the
case of plasma treated samples. The broadening combines
this second peak with the first one (green, blue and purple
lines). In the case of the corona treated sample (red line),
the second peak is strongly increased. The third peak at
289 eV does not change after corona treatment, but it is
reduced and broadened by plasma treatments.
Peak broadening corresponds to an enhancement of the
number of functional groups on the polymeric surface.
We note a different behaviour of spectra obtained from
samples after corona and Ar/O2 plasma treatments. This
could be due to the fact that treatments carried out in
plasma with inert gases introduce oxygen moieties onto
the polymer surface during a post plasma exposure of
samples to atmosphere [22,23]. An Ar containing plasma
treatment causes the breakage of some bonds, leading to
the formation of the carbon radicals and crosskinking
effects. The surface crosslinking can compete with the
oxidation process, influencing the number of covalently
bond groups introduced by plasma treatment [2,24 and
references therein]. This explains the difference of plasma
and corona results in ESCA. A detailed discussion of the
mechanisms which produce the surface modifications of
PET in Ar/O2 plasma treatments is proposed in Ref. [25].
4. Surface Energy and Wetting
The surface energy of a substrate can be evaluated by
wetting it with liquids which have well-known surface
tensions. The measurement can be obtained through the
contact angle of a pure liquid droplet on the solid sub-
strate, for instance by means of the sessile drop method.
In this method, a syringe pointed vertically down onto the
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sample surface deposits the liquid droplet. The liquid
used is referred to as the probe liquid, and the use of sev-
eral different probe liquids is required. This method is
relatively straightforward. If the solid surface under in-
vestigation is large enough, multiple droplets can be de-
posited in various locations on the sample to determine
possible heterogeneity.
To determine the surface free energy of a polymer
with its polar and dispersive portions, the contact angle is
measured with a number of test liquids and evaluated
according, for instance to Wu’s method [26]. At least
two test liquids with known surface tension and its polar
and dispersive contributions are required. Each addi-
tional liquid will increase the accuracy of the estimation.
Untreated polypropylene films, for instance, have a very
low polar portion of the surface energy. After flame and
corona treatments, the polar part is strongly increased:
experimental results are shown in Table 1.
There are other methods to determine the surface ten-
sions (Du Noüy Ring method, Wilhelmy method, Spin-
ning and Pendant Drop methods). From a practical point
of view, when it is necessary to evaluate the surface en-
ergy obtained with corona and plasma treaters, placed
on-line in converter devices for printing and lamination,
the Dyne Solution method prevails [27]. This is the pre-
ferred method to determine the surface energy of poly-
meric films treated with a specific device and is vital for
If a substrate has a low surface energy, its wettability
is poor and coating adhesion very scarce with bad final
results after printing. In-line treatments, such as corona,
flame and atmospheric plasma, have a profound impact
on the polar component of surface energy, increasing it
and modifying the surface functionalities of materials
[19,20]. These materials include polymers such as poly-
ester, polyethylene and polypropylene.
In many industries, high surface energy (above the
value of 50 mN/m) is required for satisfactory wettability,
adhesion and printability. Figure 3 shows linear trends
of the surface energy of polyester (PET), polyethylene
(PE) and polypropylene (PP) films, treated by Enercon
atmospheric plasma treatment technology, with increas-
ing power. The gas chemistries applied to form a uniform
Table 1. Surface energies and their dispersive and polar
contributions in (mN/m), evaluated according to Wu’s
method (see Ref. [26]).
Film Surface
Untreated PP 29.98 29.95 0.03
Corona treated PP 38.50 30.19 8.31
Flame treated PP 39.19 30.20 8.99
Figure 3. Linear trends of surface energies of polyester,
polyethylene and polypropylene films, treated by Enercon
atmospheric plasma treatment technology as a function of
the increasing treatment power.
high density atmospheric plasma included Ar with small
percentages of reactive gases. The gas chemistries typi-
cally applied to each of these materials are as follows: for
PP-Ar/O2, for PE-Ar/O2/Acetylene and for PET-Ar/O2.
The linear trends in Figure 3 were obtained from
measurements of surface energies by means of Dyne So-
lutions during on-line processing. As can be seen with
regard to PET, its base (untreated) surface energy is ap-
proximately of 43 mN/m. This is high, relative to poly-
olefins, since polyester is synthesised with purified tere-
phthalic acid (PTA) or its dimethyl ester dimethyl tere-
phthalate (DMT) and monoethylene glycol (MEG), all
fairly polar components. As obtained from Dyne Solu-
tions measurements, 6 W/min/m2 are required to move PET
surface energy to a value of 50 mN/m, and 14 W/min/m2
to achieve a value of 60 mN/m.
PE is a polymerised ethylene resin, containing both
carbon and hydrogen, with a resident surface tension of
around 31 mN/m and very low polarity. Although a
power density of 14.5 W/min/m2 is generally required
to achieve 50 mN/m, this is significantly less than the
average corona discharge requirement of approximately
30 W/min/m2. PP is a thermoplastic polyolefin with a
relatively high level of crystallinity and very low polarity.
It is formed by polymerising propylene with suitable
catalysts, such as aluminum alkyl and titanium tetrachlo-
ride. Its resident surface tension is about 31 mN/m, again
due to its inertness. Although its chain mobility can be con-
siderably less than other polyolefins, atmospheric plasma
can raise its surface tension to over 50 mN/m with
treatment at 18 W/min/m2. Again, this is significantly
below the corona discharge dosage of nearly 50 W/min/m2,
typically required.
5. Experiments on Adhesion
We compared also the effects of surface treatment with
plasma and corona on the printing adhesion. As in the
evaluation of the surface energy trend, the trial runs were
performed on Enercon’s Bare Roll Corona Treatment
Station, as well as its Plasma3 Atmospheric Plasma
Treatment System, resident on its same pilot line. For all
runs, a PVDC-coated polyester film was post-treated and
then printed with aqueous ink using a laser-engraved
anilox roll. The printed image provided solid (100%) ink
coverage. The calculated ink transfer (thickness) to the
film, based upon the cell volume anilox roll, is approxi-
mately 1.9 microns. The printed web drying temperature
occurred at 121C, with a web temperature of 77C. The
levels of power treatment used in this design and of the
surface energy after treatment are shown in Table 2.
A friction/peel testing equipment conforming to test
standards (ASTM-D1894,-D4521,-D3330, TAPPI-T816,
DIN-53375, BS-2782 and PSTC-1, 3, 4, 5) was used for
testing the printed polyester film. The testing protocol
employed twenty measured peel iterations for unprinted
and printed samples, each of which were corona and
APT-treated. Results in terms of the averages of these
iterations are given in Table 3.
The peel adhesion data indicated that at a power den-
sity of 10 W/min/m2, printed APT-treated PET surpassed
the peel adhesion results registered by printed co-
rona-treated polyester. Moreover, the decrease in ink
peel adhesion between unprinted and printed co-
rona-treated base material, compared to the less than a
4% decline in ink peel adhesion between unprinted and
printed APT-treated base material, suggests that the for-
Table 2. Variable levels utilised in the experimental design.
Variable APT Corona Control
Substrate 23 μm PET 23 μm PET 23 μm PET
Pretreatment None None None
Power Density 10 W/min/m2 10 W/min/m2 None
Post-Treatment 54 mN/m 46 mN/m 40 mN/m
Chemistry Helium/O2 None None
Ink Chemistry Water-based Water-based None
Table 3. Average peel adhesion for printed and unprinted
PET film after power treatment of 10 W/min/m2.
Sample Average Peel Adhesion
Corona treated, unprinted 1.28
Corona treated, printed 0.98
Plasma treated, unprinted 1.80
Plasma treated, printed 1.74
Table 4. Average peel adhesion for printed and unprinted
PET film for a surface energy of 46 mN/m.
Sample Average Peel Adhesion
Corona treated, unprinted 1.28
Corona treated, printed 0.98
Plasma treated, unprinted 1.60
Plasma treated, printed 1.45
mation of strong covalent atomic bonds on a cleaned and
uniform, homogeneously micro-etched surface may ac-
count for improved anchorage of inks.
To determine the impact on peel adhesion under con-
ditions where the surface tension created by both corona
and APT were the same, the protocols were repeated and
data reported in Table 4. This condition was established
by reducing the power density of APT to 7 W/min/m2, to
achieve 46 mN/m. This set of peel adhesion data indi-
cates that at a surface tension level of 46 mN/m printed
APT-treated polyester maintained a significantly higher
peel adhesion performance over printed corona-treated
The analysis identified that untreated flexible packag-
ing grade polyester film which was post-treated with the
APT process exhibited high levels of peel adhesion rela-
tive to the corona post-treated polyester at a power den-
sity of 10 W/min/m2. When post-treatment surface ten-
sion was equalized between APT and corona at 46 mN/m,
the APT treatment process continued to promote strong
ink anchorage relative to corona by approximately fifty
It is not easy a comparison with experimental data ob-
tained from other research groups, because experimental
set-up designs are different. We observed an agreement
with data on the strength adhesion increase, after air
plasma treatment, as reported in Ref. 23. This is a quite
interest result, because we are using industrial treatment
systems. Other data on PET, such as those in Ref. [28],
were obtained in rather different conditions and impossi-
ble to compare. These data are in any case reporting an
increase of adhesion after plasma treatment.
A different peel strength obtained after plasma or co-
rona treatment is caused by several factors. It is usually
believed that it is the creation of a wettable polar surface
responsible for the increase of strength. As observed in
[23], this is a sufficient condition for forming strong joints.
A necessary condition is to clean the surface and remove
weak boundary layers from it. The plasma cleaning is
well-known and widely used to achieve clean surfaces.
Moreover, as seen from AFM photographs, the surface
assumes, after a plasma treatment, a roughness which
increases the effective surface area suitable for adhesion
[23]. Thus, adhesion will be facilitated by all these factors.
6. Conclusions
In this paper we discussed the atmospheric plasma
treatment of polymeric surfaces. Let us note that the
plasma treatment system can be operated at low tem-
peratures and at atmospheric pressure, thereby eliminat-
ing the need for any vacuum chambers or pumps. At-
mospheric plasma provides then the advantages which
plasma technology has over the existing technologies for
surface treatment of polymers, without additional costs.
The systems we used for investigation on wetting and
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adhesion are commercial devices, then confirming the
possibility of plasma to be used in industrial converting
The surface energies of polymers treated by atmos-
pheric plasma systems have been shown to increase sub-
stantially, thereby significantly enhancing the wettability,
printability and adhesion properties. The peel tests indi-
cate that the APT process can affect better ink adhesion
than corona treatment. Our analysis on adhesion provided
evidence that flexible packaging converters utilizing aqu-
eous inks on polyester-based structures may experience
improvements in ink adhesion by employing APT-based
surface treatment systems instead of corona treaters.
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