Note: Descriptions are shown in the official language in which they were submitted.
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Polymer Surfaces with Increased Surface Energy and Appropriate
Production Method
FIELD OF THE INVENTION
The invention relates to polymer surfaces with increased surface energy and a
method
to increase the surface energy of polymer surfaces aiming at an improvement of
surface
properties, such as the adhesion of varnishes, print paint and adhesive
products, and
printability, gluability and wettability.
BACKGROUND OF THE INVENTION
Surface energy can be considered to be a measurable quantity for the linkage
forces at
surface level; it is the energy which has to be applied to divide an
infinitely expanded
solid body into two identical, half-infinite parts, keeping them at such a
distance which
prevents any interaction between the components. In a first step, fission
energy should
be applied in order to divide the solid body into two components, while a
second step
serves to keep the two components at a distance to each other allowing them to
be
shifted to new steady positions.
Subsequent wetting of polyacrylate surfaces by photons using energies of 4 -11
eV is
common knowledge and is described, for example, in Patent Specification DE 10
2008
060 906 Al. In this patent, an example (Table 2) is used for the description
of the
increase of surface energy of a UV-hardened polyacrylate-nano-composite
coating from
21 up to 23 mN/m after irradiation with photons from a 172 nm Excimer emitter.
A disadvantage is however the fact that subsequent radiation using a 172 nm
Excimer
emitter will only increase the surface energy insufficiently; it cannot
decisively improve
the polymer surface printability, gluability and wettability properties.
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In order to achieve this objective, surface energies of more than 45 mN/m and
with a
polar fraction of more than 10 mN/m are required. The polar fraction of the
surface
energy is decisive for the adhesion, printability, gluability and wettability.
Increasing the surface energy of e.g. polyacrylate and polymethacrylate
surfaces by
selecting more polar monomers and oligomer acrylates or methacrylate
components is
limited and leads to surface energies of the coating being less than 45 mN/m
with polar
fractions of less than 5 mN/m. Effective additives that can be added to the
liquid
formulations with the objective of increasing the surface energy are not
available.
The effect of corona discharges has been described e.g. in the Softal Report
102d from
SOFTAL Corona & Plasma GmbH, Hamburg. Plasma discharges that occur when an
adequately high voltage is applied to a gas filled capacitor with asymmetrical
electrodes
create conductive streamers that lead to a temporary short-circuit. In the
discharge
channel, positive and negative ions are produced with kinetic energies of up
to 100 eV
as well as electrons with energies of 12 to 16 eV. Electrons and ions with
these
energies are in a position to generate atomic oxygen and ozone in the air for
example,
an radicals and radical ions on the surface of the polymers to be treated.
Corona
systems with higher exciter frequencies are operated as streamers only have a
lifetime
of only a few 10 ns and occur with typical frequencies of 10 kHz,
The corona treatment that has been described is however not suitable to
achieve
effective and, in particular, a long lasting increase in the surface energy of
polyacrylates, polymethacrylates and vinyl polymers.
Carbon dioxide, butane, butanol and fragments of the polymer chain are created
through the thermal degradation of e.g. poly(n butyl acrylate) in the presence
of oxygen.
Monomers such as butyl acrylate, low molecular weight alkanes and alkenes,
carbon
monoxide and hydrogen are produced with a lower yield.
=
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/ V.V.Krongauz,M.T.K.Ling, Photo-cross linked acrylate degradation kinetics,
J.Ther.Anal.Calorim. (2009) 96: 715-725/ Ana/oge Prozesse der oxidativen
Degradation
der Polymeroberflache erwartet man bei Elektronen- und lonenbeschuss des
Polymeren bei Anwesenheit von Sauerstoff (Analogous processes of the oxidative
degradation of the polymer surface expected on the electron and ion
bombardment of
polymers in the presence of oxygen).
Although the corona treatment results in the polar fraction of the surface
energy of
polyacrylates and polymethacrylates being increased immediately after the
treatment,
the effect decreases sharply within a few days, so no long lasting effect is
achieved.
The reason for this effect can be the migration of low molecular weight polar
bonds on
the surface and their transition into the ambient air.
Migration is however hindered when the polar chemical groups on the polymer
chains or
on the polymer fragments are bound. Then, one achieves a long lasting increase
in the
polar fraction of the surface energy.
SUMMARY OF THE INVENTION
An objective of a preferred embodiment this invention is thus, to find
solutions that
=
create a long lasting increase in the surface energy of polyacrylate and
polymethacrylate surfaces of equal to or greater than 45 mN/m with polar
fractions of
greater than 10 mN/m.
According to a preferred embodiment of the invention, an irradiation of the
polymer
surface is carried out with photons by a Xe2 Excimer radiation source in a
nitrogen-
oxygen atmosphere with oxygen concentrations of 0.1 to 1 vol. %.
BRIEF DESCRIPTION OF THE DRAWINGS
The solution according to the embodiments of the invention is explained in
further detail
based on illustrative embodiments and a Figure:
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FIGURE 1 is a graph showing Optical emission spectrum of the Xe2 Excimer
emitter
and the penetration depth of the photons in oxygen that contains 0.1 and 1
vol. % of
oxygen.
DETAILED DESCRIPTION
According to the invention, the polymer surface is irradiated with photons
that have
energies of 6.5 to 7.8 eV. The source of the photons is placed as close as
possible to
the polymer surface.
There is a nitrogen-oxygen mix with an oxygen concentration of between 0.1 and
1 vol.
% between the source of the photons and the polymer surface. A part of the
photons is
absorbed by the oxygen. The rest of the photons reach the polymer surface.
Fig. 1
shows the depth to which the photons penetrate into the nitrogen-oxygen mix
with an
oxygen concentration of between 0.1 and 1 vol. %.
Photons with energies of between 6.5 and 7.8 eV that are absorbed by oxygen
produce
atomic oxygen through electronic excitation of the oxygen molecule. This
atomic oxygen
is converted to ozone in a reaction with molecular oxygen. It is known that
ozone reacts
with polymers and can form peroxy radicals that can initiate the degradation
of polymers
on the surface. / S.D.Razumovskl, A.A. Kefeli, G.E.Zaikov: European Polymer
Journal
Volume 7 (1971) p. 275-286/
The degradation products on the surface of the polymers contain oxygen in the
form of
polar chemical groups. These help to increase the polar fraction of the
surface energy.
Of the photons with energies of between 6.5 and 7.8 eV that reach the polymer
surface,
some penetrate 10 to 100 nm into the polymer. Primary processes are initiated
by the
electronic excitation of molecular states in polymers that finally lead to the
production of
polymer radicals.
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Polymer radicals react with the oxygen from the nitrogen-oxygen mix to form
peroxy
radicals that initiate the process of degradation on the polymer surface and
thus also
contribute to the formation of polar chemical groups and so to the increase in
the polar
fraction of the surface energy.
Excimer emitters are the preferred source of photons; for reasons of design,
they are
=
manufactured as line sources with lengths of up to 2.5 m. A source that covers
an area
is obtained by switching more than one line source together.
The following Excimer emitters are available as sources for photons
Excimer Emission wavelength Photon energy
Typical penetration depth in polymers
(nm) (eV) (nm)
Xe2 Maximum from 6.5 Maximum
<100 at 172 to 7.8 at 7.2
Preference is given to the use of Xe2 Excimer emitters as these have a broad
emission
spectrum of 160 to 185 nm. By selecting the oxygen concentration in the
emission zone,
one can ensure that adequate ozone is produced and also that adequate photons
reach
the surface of the polymer. The spectrum of the Xe2 Excimer emitter and the
penetration depth of the Xe2 Excimer photons in a nitrogen-oxygen mix are
shown in
Fig, 1 as an example.
The physical active principle, the construction and the use of Excimer
emitters are
described e.g. in:
1. B. Eliasson, U. Kogelschatz: Appl. Phys. B 46, p.229 (1988)
2. U. Kogelschatz: Pure] Appl. Chem. Vol 62, p. 1667 (1990)
3. U. Kogelschatz. Proceedings Tenth Int. Conf. Gas Discharges and their
Applications,
Vol. II, p. 972 (1992)
4. R. Mehnert, I. Janovsky, A. Pincus: UV & EB Curing Technology and
Equipment,
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Wiley- SITA, London
As Fig. 1 shows, at an oxygen concentration of 1 % e.g., the short wavelength
fraction
of the Excimer emission spectrum is absorbed to a large extent. For photons in
the
maximum of the spectrum of 172 nm, the penetration depth is approx. 5.5 cm and
increases for photons with a wavelength of greater than 175 nm to above 10 cm.
Therefore, according to the invention, the polymer surface is irradiated with
photons
from a Xe2 Excimer emitter in a nitrogen-oxygen atmosphere.
Surprisingly, it was determined that the surface energy was increased the
most, when
the photons were absorbed to an equal extent by the oxygen in the gaseous
phase as
well as on the polymer surface.
In the process according to the invention, the polymer surface is bombarded by
photons
from a Xe2 Excimer emitter in a radiation chamber that is flushed with a
nitrogen-oxygen
mix.
The duration of the irradiation can be between 0.01 and 300 sec, preferably
between
0.1 and 5 sec. The oxygen concentration can be between 0.1 and 2 vol. /0,
preferably
between 0.2 and 0.5 vol. %.
The invention generally relates to polymers of acrylate, methacrylate and
vinyl
compounds.
Examples
Example 1
An acrylate nano-composite paint (manufactured by Cetelon Nanotechnik GmbH,
Eilenburg, Sa.) that can be hardened by ultra-violet radiation is applied to a
12 pm foil of
bi-axially oriented polypropylene (BOPP) in gravure printing. The application
weight is 3
to 4 g/m2. The BOPP foil so printed is run through a UV hardening system at a
speed of
30 m/min and hardened there in the presence of oxygen. The hardening is
measured by
an infrared (ATR) spectroscope through the conversion of the olefinic double
bonds.
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The measured conversion of 93% of the double bonds means that the UV hardening
can be considered to be complete. The surface energy of the coating is
determined by a
contact angle measuring instrument manufactured by Kruss GmbH Hamburg.
The following values are obtained:
Surface energy in mN/m 41.9
Dispersible fraction 37.9 Polar fraction 3.9
After that, the coated foil is irradiated in a pilot plant manufactured by IOT
GmbH,
Leipzig, that consists of a processing fraction, an irradiation chamber with a
Xe2
Excimer emitter and a winding fraction. The radiation chamber is flushed with
a
nitrogen-oxygen mix that contains 0.4 vol. % of oxygen. The oxygen
concentration is
adjusted in a stable way by means of a needle valve and is measured using a
GSM
device manufactured by Metrotec GmbH, Kirchheim. The web speed is set at 30
m/min.
After the irradiation, the surface energy is measured again.
Surface strength in mNim
Dispersive Polar
Storage time in days Total fraction fraction
0 51 39 12
30 50 38 12
58 49 38 11
85 48 37 11
Table 1
The following values are obtained:
Surface energy in mN/m 51
Dispersible fraction 39 Polar fraction 12
Observations regarding the results of Example 1:
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After irradiation with a Xe2 Excimer emitter in a nitrogen-oxygen mix, the
surface energy
increases to 51 mN/m. The polar fraction increases significantly from 3.9 to
12 mN/m
and so exceeds the required 10 mN/m.
Example 2
The irradiated sample from Example 1 is stored under laboratory conditions.
The
surface energy is measured as a function of time. The results are given in
Table 1.
Observations regarding the results of Example 2:
After 85 days of storage, the polar fraction of the surface energy has fallen
to 11 mN/m.
The surface energy is, at 48 mN/m, however clearly higher than the required
value of 45
mN/m. The polar potion exceeds the required 10 mN/n.
Example 3
The irradiated foil with increased surface energy is laminated on a printed
sheet. Folded
boxes are made from the printed sheets. The parts of the folded boxes are
coated with
dispersion adhesive on the irradiated foil in strips and glued using a machine
with a
production speed of up to 200 m/min. Pre-treatment such as plasma treatment is
not
carried out. After a storage time of 30 sec, a peel resistance of >200 N/m is
achieved.
Result of Example 3:
The required adhesive seam resistance of the folded box samples is achieved
with the
foil that has been irradiated according to the invention. Thus,
technologically common
pre-treatment such as plasma or corona treatment can be done away with in the
folding
box machine.
Example 4
In a coating system manufactured by finitec Performance films GmbH, Berlin, an
irradiation chamber with a Xe2 Excimer emitter manufactured by 10T GmbH
Leipzig is
installed. The dose rate of the Xe2 Excimer emitter is 25 mJ/cm2max.
Optionally, a
double Xe2 Excimer emitter is also used. With that, a dose rate of 45 mJ/cm2
is
achieved, Fig. 2).
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The oxygen concentration in the irradiation chamber is set at 0.5%. A BOPP
foil with a
polyacrylate nano-composite coating corresponding to Example 1 is irradiated.
The coating system is designed for web speeds of up to 200 m/min. The
relationship
between the surface energy of the coating and the web speed at dose rates of
25 and
45 mJ/cm2 of the Excimer source are examined.
The consolidated values are given in Table 2.
Dose rate of Xe2 Excimer emitter 25 mi/cm2
Surface strength in mN/m
Web speed in m/min Dispersive Polar
Total fraction fraction
0 not irradiated 41.9 37.9 3.9
30 50.4 39 11.4
60 51.3 40 11.4
90 43.4 38.1 5.5
Dose rate of double Xe2 Excimer emitter 45 mi/cm2
Surface strength in mN/m
Web speed in m/min Non-polar Polar
Total fraction fraction
0 not irradiated 41.9 37.9 3.9
30 51.2 39.1 12.2
60 51.4 39 11.4
90 50.6 39.2 11.5
110 48.9 38.9 10.1
130 45.2 38.5 6.8
Table 2
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Observations regarding the results of Example 4:
Polar fractions of the surface energy of greater than 10 mN/m can be achieved
at web
speeds of up to 110 m/min under specific technical conditions and the use of a
double
Xe2 Excimer emitter with a dose rate of 45 mJ/cm2
The scope of the claims should not be limited by the preferred embodiments
set forth in the examples, but should be given the broadest interpretation
consistent
with the description as a whole.
