Note: Descriptions are shown in the official language in which they were submitted.
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PLASMA TREATER SYSTEMS AND TREATMENT METHODS
This application is related to allowed U.S. Patent Application No. 08/719,588,
filed 9/25/96,
which is a continuation of U.S. Patent Application No. 08/492,193, now
abandoned, the teachings of both
of which are incorporated herein by reference.
Field of the jOvention
The present invention relates to electrode designs, excitation methods, and
process conditions for
the in-line, continuous treatment of flexible webs and films for improved
surface properties, and to the
resulting films and webs having modified surface properties.
escription of the belated Art
The applications and demands for improved surface modifications of polymer
films and webs
have grown considerably in the past ten years. Increasing environmental
concerns are imposing tighter
restrictions on the levels of solvents, adhesives, and surfactants often used
with polymer films and webs.
Plasma surface treatment offers an alternative to the use of these chemicals
as well as polymer additives
and the caustic chemical treatments often used for the modification of polymer
surfaces. For many
industrial applications, high speed in-line treatment of a film or web
substrate is the desired objective.
Treater equipment having small floor signature and the capability of being
interfaced to existing
production Line equipment is often a necessity.
The high-volume plasma treatment of polymer webs or films has traditionally
been done using
either corona treatment at atmospheric pressure or low-pressure (Ps lTorr)
discharges in batch mode
operation. Conventional corona treatment using air has been around for at
lease 40 years and provides an
economical but limited technique for surface modification. Altering the
working gas of a corona treater
provides a first step in expanding the utility of conventional corona
discharge technology. Commercial
corona treaters typically operate using low-frequency sinusoidal or half sine-
wave excitation in the
frequency range of 60I-Iz to 30kHz. The electrode designs and excitation
methods used tend to limit the
discharge power densities and duty cycles attainable.
U.S. Patent No. 5,576,076 by Slootman et al. discloses a process for
depositing a layer of silicon
oxide on a traveling substrate using silane gases at high pressures, typically
atmospheric pressure. The
electrode structure is designed so that an inert gas (preferably nitrogen) is
introduced upstream of a
moving substrate. By using a combination of suction and sufficient inert gas
inlet flow, the introduction
of ambient air into a discharge region can be controlled. A small amount of a
silane gas and oxidizer
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along with an inert buffer gas are introduced into a corona-type discharge
zone. Bar-type electrodes are
excited using a sinusoidal signal at frequencies up to 60kHz. The discharge is
typically operated in a
filamentary mode, requiring a dielectric barrier on the bar electrodes. The
process yields surface
treatments considerably improved over conventional corona treatments in terms
of increased
hydrophilicity and lamination strength. The process as disclosed however uses
significant amounts of the
inert buffer gas. The hydride silane (SiH4) and an oxidant are the preferred
functionalizing gases and are
about 1% to about 4% of the inert gas. Special precautions must be taken in
gas handling since this
silane is pyrophoric and expensive.
U.S. Patent No. 5,527,629 by Gastinger et al. is a process patent similar to
U.S. Patent No.
5,576,076 with the added step of introducing a silane gas concomitantly or
after plasma treatment. A
low-frequency (20kHz) dielectric-barrier corona-type discharge is also
employed. Both of these
treatment processes could benefit from the use of a more uniform plasma to
produce improved surface
uniformity, and reduced-pressure operation to decrease gas consumption.
Low-pressure systems, which typically operate around 1 Torr or less, have the
versatility of
using essentially any gas or volatile substance as a plasma medium; however,
not all low pressure
discharge methods scale well or are capable of generating a high-power-density
discharge. In order to
avoid some of the process restrictions of low-pressure batch processes, some
recent applications of 1ow-
pressure discharges utilize annular gaps with differential pumping or roller
seals to generate a vacuum
interface. These low-pressure continuous-feed systems typically operate at or
below 1 Ton, require
multistage vacuum pumping and utilize a diffuse, low-power-density discharge
for surface modification.
U.S. Patent No. 5,314,539 by Brown et al. discloses an apparatus using
multiple roller seals to
generate a pressure interface and introduce a continuous strip of photographic
substrate or similar
material into a reduced-pressure (Ps ITorr) region for plasma treatment. A Iow-
power RF discharge is
used with air to modify the polymer substrate for improved wettability. The
low discharge current of 20
to 200 milliamps for a 12.7-cm wide film suggests that a diffuse, low-power-
density glow discharge is
generated. The electrode shapes employed also require the low-pressure (Ps
lTorr) operation. The sharp
contours or bar shapes of the electrodes employed require that a discharge be
ignited at modest voltages.
Attempts to increase the discharge power density by significant increases in
the discharge voltage would
most likely result in plasma streamers and a poorly defined discharge. Because
of the required low-
pressure operation, multiple roller-seal stages are required, complicating the
device's construction and
maintenance.
U.S. Patent No. 5,529,631 by Yoshikawa et al. discloses an apparatus using
roller or gap seals to
introduce a continuous sheet-like material into a high-pressure plasma
treatment zone. The discharge is
operated at essentially atmospheric pressure with helium gas and a few percent
of a working gas as the
discharge gas. Planar, dielectric-covered electrodes are excited with a
sinusoidal signal of SOHz or
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greater. The system's use of significant amounts of helium would most likely
compromise its cost
effectiveness for large-volume, high-speed treatment of films or webs. Many
applications for films
require single-side treatment. This type of treatment would be difficult to
obtain with these
embodiments.
The above patent disclosures involve apparatus or processes that rely on the
substrate being
exposed directly to plasma species generated in the plasma-discharge zone.
Polymer surfaces can also be
modified by convecting plasma species and metastables out of the production
zone and onto the polymer
surface. This method is most effectively used at pressures below one Ton due
to the rapid collisional
recombination of ions that occurs at high gas pressures. The use of a
microwave-generated plasma (f =
2.45GHz) to treat polymers films located downstream of the plasma source is
discussed in Foerch et al.,
"Oxidation of polyethylene by remote plasma discharge: a comparison with
alternative oxidation
methods," J. Polym. Sue. A. 28, pp. 193-204 (1990), and Foerch et al., "A
comparative study of the
effects of remote nitrogen plasma, remote oxygen plasma, and corona discharge
treatments on the surface
properties of polyethylene," ,[. Adhesion Sci. Technologrr 5, pp. 549-564 (
1991 ). Surface levels of
oxygen up to 30% and surface levels of nitrogen up to 40% were reported. The
system employed
operated at low pressure (P~lTorr) and would be difficult to scale for wide
films.
U.S. Patent No. 4,937,094 by Doehler et al. discusses the use of a microwave-
or RF-generated
plasma to generate a high flux of metastable species which in turn are used to
transfer energy to a
remotely introduced precursor or etchant gas. Metastable species are generated
by using a gas jet to
convect active plasma species out of the microwave- or RF-discharge zone for a
sufl:<ciently long path
that the ion species recombine to form metastables. Long-lived metastables are
convected by the pressure
differential, and are either used to collide with a remotely introduced gas
with low-level ionization
resulting or applied directly to a substrate material. The embodiment is
directed primarily to amorphous-
silicon deposition and etching techniques for semiconductor applications, and
small-area treatments.
Atmospheric-pressure remote-plasma treatments have been reported by Frierich
et al., "The
improvement of adhesion of polyurethane-polypropylene composites by short time
exposure of
polypropylene to low and atmospheric pressure plasma," J. Adhesion Sci.
TechnoloQV 9, pp. 575-598
(1995), using three different discharge methods: spark jet plasma, arc jet
plasma, and corona jet plasma.
These discharges used low-frequency a.c., high-current d.c., and high-voltage
a.c., respectively, to
generate plasma discharges. Good lap-shear strength was obtained on laminated
polypropylene-
polyurethane samples with exposure times as short as 0.1 seconds. Due to gas-
flow considerations and
the electrode designs used, use of specialty gases and uniformity of treatment
become concerns.
U.S. Patent No. 5,458,856 by Maurepas et al. discloses a coaxial discharge
used for the
production of ozone, supplying a COZ laser or the production of atmospheres
for the nitriding of metals.
The electrodes are excited using a low-frequency sinusoidal signal in the
frequency range 20kHz to
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60kHz. Ozone or excited metastables are converted radial out of the tubular
gas passage formed by the
coaxial geometry. The system is similar to the coaxial geometry of ozone
generators used for water
purification in Europe except with radial and azimuthal gas flow employed. The
sharp edges and
discontinuities of the design are not amicable to the generation of high-
power, uniform-plasma
discharges. No mention is made of treating web or film materials using the
active-plasma species and
metastables converted by means of sufficient gas flow.
The present invention provides improvements over the teachings of the prior
art. Further aspects
and advantages of this invention will become apparent from the detailed
description which follows.
SUMMARY OF THE INVENTION
The present invention is directed to continuous-feed plasma treater systems
designed to treat
continuous substrates, such as webs or films, by continuously feeding the
substrates through an enclosure
having a plasma discharge that alters the substrate's surface properties in
some desirable fashion. The
plasma discharges are generated by one or more electrode assemblies housed
within the enclosure. In
general, the plasma treater systems of the present invention have one or more
cylinder-sleeve electrode
assemblies and/or one or more cylindrical cavity electrode assemblies. In one
particular embodiment, the
plasma treater system has one of each type of electrode assembly.
A cylinder-sleeve electrode assembly comprises a cylinder electrode and a
sleeve electrode
positioned with its concave face facing and substantially parallel to the
cylinder electrode to form an
annular gap between the outer surface of the cylinder electrode and the inner
surface of the sleeve
electrode. The electrode assembly is adapted to be excited to generate a
plasma within the annular gap to
form a primary plasma discharge zone for exposure of the plasma to a
substrate.
A cylindrical cavity electrode assembly comprises ( 1 ) a cavity electrode,
having a cylindrical
bore and a wall slot running parallel to the cylindrical bore and exposing the
cylindrical bore; (2) a
cylinder electrode coaxially positioned within the cylindrical bore of the
cavity electrode to form an
annular gap between the outer surface of the cylinder electrode and the
surface of the cylindrical bore, the
annular gap forming a primary plasma discharge zone; and (3) a treater drum
positioned adjacent to the
wall slot of the cavity electrode. During operations, the substrate is
translated past the wall slot by the
treater drum and exposed to plasma species that are converted from the primary
plasma discharge zone to
the wall slot.
Plasma treater systems of the present invention can be used (a) to treat
substrates in an efficient
cost-effective manner and (b) to produce treated substrates having superior
surface properties as
compared to those generated using prior-art systems, such as corona-type
discharge systems.
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Other aspects, features, and advantages of the present invention will become
more fully apparent
from the following detailed description, the appended claims, and the
accompanying drawings in which:
Fig. 1 is a schematic drawing of a plasma treater system, according to one
embodiment of the
present invention, for the continuous treatment of flexible web or film
substrates;
Fig. 2 shows a midplane cross-sectional side view and a midplane cross-
sectional end view of a
cylinder-sleeve electrode assembly, according to one embodiment of the present
invention;
Fig. 3 is a schematic drawing of a plasma treater system, according to an
alternative embodiment
of the present invention, for the continuous treatment of flexible web or film
substrates;
Fig. 4 shows a midplane cross-sectional end view of the cylinder-sleeve
electrode assembly of the
plasma treater system of Fig. 3;
Fig. 5 shows a midplane cross-sectional side view and a midplane cross-
sectional end view of a
cylindrical cavity electrode assembly, according to one embodiment of the
present invention;
Fig. 6 shows a midplane cross-sectional end view of the lower portion of a
cylindrical cavity
electrode assembly, according to an alternative embodiment of the present
invention;
Figs. 7 and 8 illustrate circuit diagrams that model the components and plasma
involved in a
resonant RF discharge;
Fig. 9 is an oscillograph trace of an applied low-frequency sinusoidal voltage
and the resulting
voltage response of a photo multiplier tube imaging the plasma discharge
generated by the applied
voltage;
Fig. 10 is an oscillograph trace of an applied asymmetric pulsed voltage and
the resulting voltage
response of a photo multiplier tube imaging the plasma discharge generated by
the applied voltage;
Fig. 11 is an oscillograph trace of an applied high-frequency resonant RF
sinusoidal voltage and
the resulting voltage response of a photo multiplier tube imaging the plasma
discharge generated by the
applied voltage;
Fig. 12 is a graph illustrating the scaling of discharge power density verses
applied electrode
voltage for the cylinder-sleeve electrode geometry using the resonant RF
excitation;
Fig. 13 shows a midplane cross-sectional end view of a twin-drum plasma
treater for laminating
films, according to one embodiment of the present invention;
Fig. 14 illustrates the lifetimes of some various reaction products and ion
species for a micro
discharge modeled in air;
Fig. 15 presents experimental scaling of the surface oxygen and nitrogen
deposited on
polypropylene film using the cylinder-sleeve electrode geometry with
asymmetric pulsed voltage
excitation;
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Fig. 16 presents experimental scaling of the surface oxygen and nitrogen
deposited on
polyethylene film using the cylinder-sleeve electrode geometry with asymmetric
pulsed voltage
excitation;
Table 1 lists the results of a study directed to cleaning surface additives
from polymer films;
Table 2 lists plasma treatment conditions used to treat two polymer films to
remove surface
additives which inhibit ink adhesion;
Table 3 lists the treatment conditions for a lamination study;
Table 4 lists plasma treatment conditions used to treat polyethylene films
coated with various
levels of ethylene vinyl acetate and peel-strength data for the laminated
plasma-treated film;
Table S lists treatment conditions and contact angles for the treatment of
polyethylene using He +
CF, and He + CZF6 mixtures;
Table 6 lists plasma treatment conditions used to treat linear low-density
polyethylene with a
low-power direct-exposure plasma using the cylinder-sleeve electrodes followed
by indirect plasma
exposure using the cylindrical cavity electrode assembly;
Table 7 lists plasma treatment conditions used on polyethylene film using a
silicon tetrachloride
m fixture;
Table 8 lists plasma treatment conditions used on polyethylene films using a
dichlorosilane gas
mixture;
Tables 9 and 10 list performance data for a 20 gr/mz polypropylene, spunbond
treated using the
RF resonant discharge at a reduced pressure of ~55 Torr; and
Table 11 summarizes the operating parameters of the discharge techniques of
the present
invention as compared to conventional corona-discharge technology.
DETAILED DESCRIPTION
The present invention concerns electrode designs, excitation methods, and
process conditions
used for generating uniform, nonequilibrium, high-power-density plasma
discharges suitable for the
continuous surface modification of polymer webs and films. The invention also
concerns films and webs
which have surface properties which have been modified to give them
characteristics different from what
they possessed prior to treatment.
Two electrode geometries using either of two excitation methods are disclosed.
The first
electrode employs direct plasma exposure whereby a film or web-like material
is translated through the
discharge zone and exposed to plasma species. The second electrode structure
provides "remote" plasma
treatment. A film or web-like material is external to the plasma discharge
zone and exposed "indirectly"
to plasma-generated species by using gas flow or electric fields to convect
plasma species onto the
substrate's surface. Both electrode designs can be electrically excited using
either an asymmetric voltage
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pulse or configured as part of a resonant circuit and excited using a
sinusoidal RF signal. Pulsed-voltage
excitation may require one or both electrode surfaces to be covered by a
suitable dielectric covering. At
reduced pressure operation of P s 60 Ton, however, the pulsed-voltage
excitation can typically be used
without dielectric coverings. The resonant RF excitation may be used with
electrodes configured with or
without a dielectric barrier depending on the operating conditions desired.
A small-volume, high-power-density, plasma discharge may be generated using a
wide variety of
gases or gas mixtures. Treatment is generally performed at pressures ranging
from at or near atmospheric
pressure to the modest rough vacuum of about 10 Torr. The electrode designs
and excitation methods
disclosed produce improved plasma power densities over existing corona
treaters, and scale well for the
treatment of wide (Z 1 meter) but thin (s S millimeter) substrates.
A wide variety of substrates may be treated in accordance with the invention.
The substrates may
be fibers, sheets, films or webs (woven or non-woven), permeable (or porous)
to fluids (gases, liquids) or
non-porous. The substrates can be natural or synthetic materials, like
polymers. Natural materials
include, for illustration, cotton, wool, leather, paper, etc., which may be
treated as such or as components
of laminates, composites, or other materials. A great variety of organic
synthetic polymers can be treated
such as acetals (polyoxymethylenes), polyamides, polyacrylates, polyoiefins
(like polyethylene (high and
low-density; branched polyethylene), polypropylene, polybutylene, ctc.),
polycarbonates, polyesters (like
polyterephthalates, polybutylene terephthalates (PBT),
polyethylenetherephthalete (PET),
polyetherimide, polycarbonates, polyphenylene sulfide; ethylene-ethyl
acrylate; ethylene acid copolymer;
ethylene-vinyl acetate, ethylene - methylacrylate; ethylene vinyl alcohol;
polyimides; polymethyl
pentane; polyphenylene oxide, polyphenylene sulfide; styrenic resins (ABS);
acrylic-styrene acrylonitrile;
polystyrene; polyvinylchloride vinylidine chloride, etc.
Film-forming polymers and polymers which lend themselves to form fibers
(filaments) and webs
are especially desirable for certain applications. The polymers may be
homopolymers or copolymers.
The polymers may be thermoplastics or thermosets. Suitable polymers may be
selected from Modern
Plastics, Encyclopedia, P.O. Box 602, Hightstown, NJ 08520.
Numerous polymers are being used for an increasing variety of applications
such as food
packaging, electrical insulation, advanced composites, etc. Polymers are
selected for a given application
on the basis of their physical, electrical, and chemical properties, thermal
stability, coefficient of thermal
expansion, chemical resistance, and others properties.
The invention can be used to modify or improve the surface properties of the
substrate, for
instance, wettability (hydrophilicity), hydrophobicity, reduce the static
properties of the surface of the
substrate, to improve adhesion of other or same materials to the surface of
the substrate, and change other
properties. For plasma modification of polymer surfaces, see IBMJ. Res.
Develop. Yol. 38, No. 4, July
1994, Plasma Modification of Polymer Surfaces for Adhesion Improvement, F.D.
Egitto and L.J.
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Matienzo, which is incorporated herein by reference. Depending on the
thickness of the substrate, the
invention can also be used to modify or improve the bulk properties of the
substrate (e.g, tensile
strength).
Applications of the invention which are of particular interest include: the
use of a high-power-
density RF discharge to remove slip agents from polymer films to promote ink
adhesion for printing;
polymer films with surfaces activated using a high-pressure plasma discharge
that are capable of being
pressure laminated without the use of an adhesive coating; the use of high-
pressure pulsed plasma
discharge to produce film surfaces with higher levels of oxygen and/or
nitrogen than can be obtained
using conventional corona treatment technology; polymer films with fluorinated
surfaces having
increased hydrophobic properties; polymer films having low advancing contact
angle produced by
grafting acrylic acid to the polymer surface; polymer films treated with
silane gases having potential anti-
fog application; and the treatment of web substrates for improved
hydrophilicity.
The substrates to be treated may be spunbond, meltblown, needle punched, hydro-
entangled,
woven, or a combination of these techniques. Polymer surfaces can be
effectively treated for a variety of
applications. Surfaces can be cleaned of process lubricants or waxes and
activated for improved
printability or lamination. The surfaces of inert polymers such as
polypropylene and polyethylene can be
chemically functionalized to alter hydrophilic, hydrophobic, and/or adhesion
properties. Polymer
surfaces can be activated and simultaneously or subsequently exposed to a
monomer for grafting. Barrier
coatings such as silicon dioxide can deposited directly during plasma
exposure.
A wide variety of gases, volatile vapors, and liquids volatile under the
treating conditions may be
used as discharge media to yield various surface treatment on both film and
web substrates. Some of
these include: inert gases like argon and helium, oxygen, air, hydrogen,
nitrogen, nitrous oxide, nitric
oxide, ammonia, chlorine, sulfur dioxide, carbon dioxide, sulfur hexafluoride,
tetrafluoromethane,
hexafluoroethane, perfluoropropane, acrylic acid, and substituted silanes like
dichlorasilane, silicon
tetrachloride, and tetraethylorthosilicate. The substances silicon
tetrachloride, tetraethyiorthosilicate, and
acrylic acid are volatile liquids at ambient pressure and temperature, but
readily become vaporous at
reduced pressure or with slight warming. The gases and vapors listed above are
most often used as
mixtures in combinations of two or more to produce a specific type of surface
treatment. Examples are
presented as preferred embodiments when using the resonant RF excitation
method. It is preferable to
adjust the gas mixture to match the operating pressure, so that, at reduced
pressures as in the range of
about 150 Torr or lower, any gas may be used, whereas, at pressures in the
range of 150 Torr and higher,
it is preferable to increase the proportion of inert or of the lesser active
gases.
A treater system consists of one or more electrode assemblies housed within an
enclosure
maintained at a pressure ranging from at or near atmospheric pressure down to
the rough vacuum of about
Ton. The enclosure is equipped with nip or roller seals to allow for the
continuous movement of a
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thin web or film through the enclosure, yet minimize the introduction of
ambient air into the plasma
discharge zone or the escape of a working gas. The enclosure serves as an
electrostatic shield and to
facilitate the recovery of the working gas. Recovered gases can be
dehumidified and reused as a
discharge gas, or forced through web materials to purge entrained gases and
moisture.
Reduced-pressure operation of the discharge over the pressure range from about
10 Torr to about
I Atmosphere (e.g., 760 t 50 Torr) has some specific advantages over operation
at strictly atmospheric
pressure (P=760 Torr) or traditional low-pressure plasma treatments typically
operated below one Torr.
A rough vacuum in the range of about 10 Torr to about 200 Torr is easily
obtained using single-stage
vacuum pumping with either a mechanical pump or steam ejector. Rotary-vane and
screw-type
mechanical pumps are efficient and economical means for generating this level
of rough vacuum. A
single set of nip or roller seals can be used for rough-vacuum operations as
opposed to the series of
multiple seals needed for low-pressure operations. This pressure range,
however, still allows the
discharge considerable immunity to contamination by leakage gases or gases
imported into the treater by
the substrate. At rough-vacuum operation, the required supply-gas flow rate is
significantly reduced
compared to strictly atmospheric operation allowing for the economical use of
more expensive specialty
gases. The use of toxic or combustible gases can also be more efficiently
managed at rough vacuum
operation. Low-pressure plasma treatments operating at pressures of Ps I Ton,
have many of these same
advantages, but require considerable vacuum pumping for use as a continuous-
feed treatment system.
Most low-pressure systems utilize diffuse "glow" discharges with power
densities of less than a watt per
cubic centimeter. The present embodiments when using the RF-resonant
excitation are capable of power
densities in excess of fifty watts per cubic centimeter. This allows for a
smaller discharge zone, and
hence a smaller treater.
The first electrode design consists of a cylindrical electrode positioned next
to an outer sleeve
electrode to form a narrow annular gap in which a nonequilibrium plasma
discharge is excited. The
sleeve electrode is machined with gas flow ports for the radial flow of a
supply gas into the discharge
zone and internal fluid passages for the temperature regulation of the
electrode and supply gas. The edge
contours of the sleeve electrode are machined to provide a smooth transition
from the uniform, high-
strength electric field of the discharge zone within the annular gap, to a
weak electric field beyond the
discharge zone. A sheet-like web or film is continuously translated through
the plasma discharge zone
and exposed directly to the plasma species by being positioned against the
outer surface of the cylinder
electrode which is rotated and temperature-regulated. Depending on the
excitation method and desired
operating conditions, one or both electrode surfaces may be covered with a
dielectric material. A
preferred embodiment illustrated in the present disclosure consists of using a
dielectric barrier on the
outer surface of the cylinder electrode.
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The second electrode design is an extension of the gridded-electrode assembly
detailed in PCT
application US96/20919 to cylindrical geometry. The design consists of a
cylindrical electrode coaxially
positioned within the bore of a cavity electrode with the cavity electrode
having a slot machined in the
side wall of the cavity. The cylinder electrode may be positioned off axis of
the cavity electrode's bore
toward or away from the wall slot. A plasma discharge is generated in the
annular region between the
wall of the cavity electrode and the outer circumference of the cylindrical
electrode. Long-lived plasma
species are convected through the wall slot by means of a gas flow introduced
into the cavity electrode.
External to the cavity electrode is positioned a temperature-controlled
treater drum parallel to and in the
vicinity of the cavity wall slot. A web or film positioned on the outer
diameter of the treater drum is
translated past the wall slot and exposed to the plasma species. The treater
drum can be electrically
biased using either a do bias, an asymmetric voltage pulse, or a low-voltage
RF signal to facilitate
improved exposure of the substrate. In an alternative embodiment of the second
electrode design, the
cavity electrode is designed in two pieces to simplify fabrication and to
allow the use of distributed shunt
inductors.
The asymmetric pulsed-voltage excitation for use with the above electrode
designs is described in
PCT application US96/20919. A positive or negative voltage pulse having a rise
time z, with m 1 psec
typically, amplitude S00 volts or greater, and pulse repetition rate 1 kHz to
100 kHz is applied to one of
the discharge electrodes with the other electrode grounded. This type of
voltage signal has the advantage
of producing higher power-density discharges than a uniform sinusoidal signal
having a frequency
comparable to the pulsed signal's repetition rate. At atmospheric pressure,
this excitation method can be
used to produce a low-duty-cycle, uniform discharge with essentially any gas
or gas mixture. At high-
pressure operation (PZ300 Torr), turbulent gas flow within the discharge zone
generated by a sufficient
supply-gas flow rate, tends to delay the onset of filamentary discharges.
Reduced-pressure operation
using this excitation method allows higher pulse-repetition rates to be used
without the formation of
filamentary or streamer discharges. The asymmetric voltage waveform also
allows the substrate to be
preferentially exposed to charged plasma species of one charge or the other
depending on the voltage bias
and electrode configuration used.
The resonant RF-voltage excitation is also described in PCT application
US96/20919. This
excitation method uses a variable inductor in shunt with the discharge
electrodes to form a resonant LC
circuit. The resonant radian frequency w~ for the parallel LC network is given
by: w~ _ (LSCo)'', where LS
is the inductance of the shunt inductor and Co is the effective gap and sheath
capacitance of the electrodes
and plasma discharge. The variable shunt inductor typically takes the form of
a wound coil which can be
compressed or elongated to increase or decrease the coil's inductance. At
moderate to high pressure
(about 10 Torr s P s about 1 Atmosphere), the impedance of the electrode gap
and plasma dielectric is
predominately capacitive and resistive over the frequency range 1 MHz to 100
MHz. When coupled to
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the external shunt inductor or a distributed shunt inductance, and this
inductance is tuned for resonance at
the frequency of a supply generator, the resulting circuit has predominately a
real impedance. This
impedance can be effectively matched to that of the RF generator using a
suitable matching network. A
balanced pi-matching network is preferred in that it applies equal voltages to
the electrodes and provides
some immunity from stray capacitance effects. The real impedance is desirable
both for e~ciency in
power transfer and the generation of high voltages necessary for gas
breakdown. The formation of a
resonant discharge increases the discharge's stability to minor changes in
boundary conditions due to a
translating web or film. When properly excited between narrow discharge gaps,
a stable high-current
anomalous-alpha discharge or a gamma discharge is generated. (See Y.P. Razier
et al., Radio-Frequency
Capacitive Discharges, CRC Press {1995), or Y.P. Razier, Gas Discharge
Phxsics, Springer-Verlag Berlin
Heidelberg ( 1991 ).) The normal current density on the electrode face is
typically greater than 0.01
amp/cmZ. The discharge tends to fill the electrode gap uniformly with an
ignited plasma having a 100%
duty cycle. Power densities ten times or greater than those obtained using the
asymmetric pulsed voltage
can be obtained. For stable operation at atmospheric pressure, this type of
excitation typically requires
the use of an inert or Nobel gas, such as helium or argon, and a few to
several percent of a working gas,
such as oxygen or carbon dioxide. By reducing the discharge pressure to
nominally Ps 150 Torr,
essentially any gas or gas mixture can be used, and a stable, uniform high-
power-density plasma
discharge produced. The upper frequency of operation can be extended from 30
MHz to 82 MHz. Since
the power density for this type of discharge scales as c~u'f', and the
electron number density as w°°, where
w is the radian frequency of the exciting voltage, using higher frequencies
has specific advantages.
Diffculties arise however with the physical scaling of the discharge at higher
frequencies due to
wavelength effects. The use of distributed inductances and proper feed
techniques are then required to
produce a uniform discharge along the electrodes length.
I. High-Pressure Plasraa Treater System
Fig. 1 is a schematic drawing of a plasma treater system 1, according to one
embodiment of the
present invention, for the continuous treatment of flexible web or film
substrates. The flexible substrate
2 is fed into an enclosure 3 which houses one or more treater heads. Fig. 1
illustrates two different types
of treater heads 4 and 5, housed within enclosure 3. Roller-seal assemblies 6
are used to provide feed-
through mechanisms for substrate 2 to enter and exit enclosure 3, while
minimizing the introduction of
ambient air into enclosure 3 or the escape of process gases from within
enclosure 3. Substrate 2 is guided
past treater heads 4 and 5 by means of guide rollers 7 and by having substrate
2 positioned against
cylindrical electrodes 8 and 9. In the present illustration, treater head 4
and cylinder electrode 8 comprise
the "cylinder-sleeve electrode assembly" and treater head 5 and cylinder
electrode 9 comprise the
"cylindrical cavity electrode assembly" These assemblies are disclosed in
detail in subsequent
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embodiments. Plasma exposure of substrate 2 occurs at treater head 4 when a
working gas in gap 10
electrically breaks down with the application of a strong electric field
caused by an applied voltage
difference between treater head 4 and cylinder electrode 8. This voltage
difference can be generated by
using either the asymmetric voltage pulse from pulse power supply 11 and
matching transformer 12, or
the sinusoidal RF voltage generated by RF power supply 13 and matching network
14. In a similar
manner, a second plasma exposure may be performed at treater head 5 with the
plasma excited by either
the asymmetric pulsed voltage from supply 11 or the RF voltage from supply 13.
Supply gases 15 are pressure regulated, metered, and mixed (16) and then
supplied to treater
heads 4 and 5 through lines 17 and 18, respectively, to become the plasma
medium. The pressure P
within enclosure 3 is regulated anywhere from at or near atmospheric pressure
down to a rough vacuum
of about 10 Torr by means of gas metering 16 and exhaust or vacuum pump I9. If
necessary, exhaust
gases from vacuum pump 19 can be treated by means of suitable gas treater 20.
The working gas within
enclosure 3 can be partially recovered by means of blower 2I, dehumidified in
accumulator 22, and
recirculated to treater head 4 and/or 5 for reuse as a plasma medium.
Recovered working gas can also be
used to purge (23) entrained and adsorbed gases from web or film substrates 2.
The accumulator 22,
treater heads 4 and 5, and electrodes 8 and 9 can be temperature regulated by
means of the chiller/heater
24.
Different types of roller-seal assemblies 6 can be used as a feed-through
device for a thin film or
web substrate 2. A single three-roller feed-through assembly consisting of two
noncompliant rollers and
a single compliant roller can be used to provide the desired pressure
differential; however, the assembly
in Fig. I is detailed as a preferred embodiment. This assembly has four
rollers: two noncompliant metal
rollers 25 with wiper seals 26 along the length of the rollers 25, and two
compliant rollers 27, each rolling
against the two noncompliant rollers 25. This configuration produces a cavity
28 between the four rollers
which can be evacuated by means of exhaust pump 19 through line 29. By
maintaining the pressure in
cavity 28 less than the pressure in enclosure 3, the leakage of ambient air
into enclosure 3 during
reduced-pressure operation can be significantly decreased. Each wiper seal 26
is adjustable and can be
made using a chemically resistant material such as a glass-reinforced Teflon.
The noncompliant rollers
25 should be hardened and ground to minimize wear of wiper seal 26. This will
also reduce desorption
from the surfaces of rollers 25, which continually enter and exit the
controlled environments of enclosure
3 and cavity 28 and the ambient air.
Overall system control and data acquisition can be computer controlled (30).
High-speed
operation typically requires tension control and motor drives (not shown) on
supply reel 3I, take-up reel
32, guide rollers 7, and possibly each roller feed-through assembly 6. These
drives should be
synchronized for start-up, run, and stop operation. For some applications, any
stretching of the substrate
during or after plasma exposure can produce deleterious effects. When not used
as an in-Line treater, the
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primary enclosure 3 can be fitted with a secondary enclosure 33. This
enclosure can be purged with an
inert gas, for example, to minimize post-treatment reactions that may occur
with ambient air.
Those skilled in the art will understand that, for operations at atmospheric
pressure (e.g., with
pulsed voltage), an enclosure is not necessarily needed.
II. Cylinder-Sleeve Electrode Assembly
Fig. 2 shows a midplane cross-sectional side view and a midplane cross-
sectional end view of a
cylinder-sleeve electrode assembly, according to one embodiment of the present
invention. The electrode
configuration has a cylindrically shaped electrode and metal sleeve electrode
with contoured edges and is
suitable for use in generating a uniform plasma discharge over the gas
pressures range of about 10 Torr s
P s about 1 Atmosphere. The sleeve electrode 4 is positioned so that its
concave face is facing and
parallel to the cylindrical electrode 8. Sleeve electrode 4 is positioned
typically within a few millimeters
of cylinder electrode 8 so that a narrow, uniform annular region 10 formed.
The sleeve electrode 4 and
cylinder electrode 8 are held in position relative to each other by means of a
support structure not shown.
This structure electrically isolates the two electrodes 4 and 8 as well as any
external structures such as the
enclosure. It is preferable that this structure provide a means of adjusting
the relative position of
electrodes 4 and 8, so that annular gap 10 may be varied by several
millimeters.
The metal sleeve electrode 4 machined with an internal radius of curvature
(34) slightly greater
than the outer radius of cylindrical electrode 8. This difference in radii may
be a few millimeters for
strictly film treatment or several millimeters for treatment of thicker web
materials. Gas flow passages
35 and fluid passages 36 are machined into the body of sleeve electrode 4
parallel to its length. Small
holes 37, typically #65 or larger, are drilled radially from the concave face
of the sleeve electrode 4 to the
gas passages 35. The edge regions 38 of sleeve electrode 4 are machined to
approximately an elliptic or
hyperbolic contour to provide a smooth transition of the electric field within
the narrow annular gap 10 to
the outside of sleeve electrode 4. Annular gap 10 is the primary plasma
discharge zone, with some
plasma extending to the contoured region 38.
The cylinder electrode 8 is machined with the center bored concentric to the
outer diameter and
fitted with bearings 39 and shaft seals 40 to accommodate a support shaft 41.
The support shaft 41 is
machined with an inlet passage 42 and outlet passage 43 to accommodate the
flow of a temperature
regulating fluid. Clearance is machined in the bore of the cylinder electrode
8 so that an annular passage
44 is formed to allow the flow of the temperature-regulated fluid between
inlet passage 42 and outlet
passage 43. The outer diameter of cylinder electrode 8 may be fitted with a
dielectric material 45, either
in the form of a dielectric sleeve or dielectric coating. A suitable
dielectric coating may also be either
flame- or plasma-sprayed onto the outer diameter of the cylinder electrode 8
or the sleeve electrode 4.
When a dielectric sleeve is used, the cylinder electrode 8 should be machined
with sufl'icient clearance to
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accommodate thermal-expansion differences between the metal-cylinder electrode
8 and dielectric sleeve
45. The cylinder electrode 8 can be machined with a set of "O"-ring grooves 46
and fitted with "O" rings
47, and a suitable oil used to fill the clearance gap 48 between the cylinder
electrode 8 and dielectric
sleeve 45. This will provide thermal contact between outer dielectric sleeve
45 and temperature-
regulated cylinder electrode 8, and prevent a plasma discharge from occurring
within this clearance gap.
Guide rollers 7 may be employed to guide and position substrate 2 securely
against the outer
diameter of cylinder electrode 8. Contact between substrate 2 and electrode 8
should be made before
annular gap 10 and contour region 38. Web materials in particular are
susceptible to thermal damage or
burn through which can be minimized with thermal contact to cylinder electrode
8 and sufficient gas flow
from outlet holes 37.
The embodiment illustrated in Fig. 2 can be modified for particular
applications without
significantly departing from the scope of the design. A wide treater will
require a larger diameter
cylinder electrode 8 than would a narrow treater. Thin-substrate treatments
may require cylinder
electrode 8 or guide rollers 7 to be driven in order to reduce the tension of
substrate 2. If the treatment
options allow cylinder electrode 8 to be used without a dielectric sleeve 45,
or if a sprayed dielectric
coating is employed, an elliptic or hyperbolic contour similar to 38 can be
machined onto cylinder
electrode 8. This contour should, however, extend into the region of cylinder
electrode 8 covered by
sleeve electrode 4.
For RF-resonant excitation of short-electrode structures (Ls0.5 meter),
electrical connection to
sleeve electrode 4 and cylinder electrode 8 is best facilitated by connecting
a high-voltage-supply lead
separately to each end the electrode pair and the variable shunt inductor to
the opposite ends of the
electrode pair. For longer electrodes and higher-frequency operation (f Z
13.56 MHz), two sleeve
electrodes 4 can be positioned azimuthally around a single cylinder electrode
8. A sequence of n variable
inductors can be connected between the two sleeve electrodes. Since these
inductors are in parallel, the
required inductance of each shunt inductor will be n times the required
inductance for resonance with the
shunt capacitance generated by the two sleeve electrodes. This shunt
capacitance will be the series
combination of the gap capacitances of the two sleeve electrodes. Each high-
voltage connecting lead
from the pi-matching network should be branched into two or more leads of
equal length, and connected
with uniform spacing to each sleeve electrode. The shunt variable inductors
can be partially tuned
independently to provide a more uniform voltage distribution along the
discharge gap.
Electrode excitation using the asymmetric pulsed voltage is simpler than for
the resonant RF-
voltage excitation since wavelength effects are less important. For pulsed
voltage operation, the
cylindrical electrode 8 is usually grounded and the sleeve electrode 4 is
excited with the voltage pulse.
Electrical connection is generally made in the center of the backside of
electrode 4, or to a metal support
bar mounted on the backside of electrode 4.
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Fig. 3 is a schematic drawing of a plasma treater system, according to an
alternative embodiment
of the present invention, for the continuous treatment of flexible web or film
substrates. This
embodiment is based on a variation of the cylinder-sleeve electrode assembly
of Fig. 2, in which the
sleeve electrode comprises four electrodes, electrically connected in two
pairs by shunt inductors. In this
embodiment, the enclosure houses a single cylinder electrode with the two
pairs of sleeve electrodes
excited in the resonant ItF excitation mode. The substrate enters and exits
the enclosure through a single
roller-seal assembly. Fig. 4 shows a midplane cross-sectional end view of the
cylinder-sleeve electrode
assembly of the plasma treater system of Fig. 3.
III. Cylindrical Cavity Electrode Assembly
Fig. 5 shows a midplane cross-sectional side view and a midplane cross-
sectional end view of a
cylindrical cavity electrode assembly consisting of three separate electrodes
for the production of two
distinct regions of plasma for plasma treatment of film or web substrates,
according to one embodiment
of the present invention. This configuration is an extension and improvement
of the grid-electrode
geometry presented in PCT application US96/20919. At moderate to high gas
pressures (about 10 Torr <_
P s about 1 Atmosphere), strong electric fields are required to initiate a
plasma discharge; however, once
a discharge is initiated, a weaker electric field can be used to sustain the
plasma or bias long-lived plasma
species. The following electrode geometry is designed to produce two distinct
regions of electric feld: a
primary region of strong electric field were a plasma discharge is generated
and a secondary region of
weak or no electric field where a plasma is sustained or long-lived plasma
species are convected and used
to treat a polymer substrate.
Cavity electrode 5 having a cylindrical bore is fitted with a coaxially
positioned cylinder
electrode 50. A portion of the cavity electrode's wall is machined away to
form a slot Sl running parallel
to the cavity electrode's bore. The internal-cylinder electrode 50 may be
positioned off axis of the bore of
cavity electrode 5 toward or away from wall slot 51. The internal edge 52 of
the cavity electrode 4 near
wall slot 5i is machined with a smooth contour (e.g., elliptic or hyperbolic
in shape) to provide a gradual
transition of the electric field established within the annular gap 53 between
cavity electrode 5 and
cylinder electrode 50. With the application of a suitable high-voltage signal
applied between the cavity
electrode 5 and the cylinder electrode 50, a working gas within annular gap 53
is made to break down and
form a plasma discharge. In Fig. 5, the cylinder electrode 50 is displaced
toward wall slot 51, so that the
plasma discharge will occur near wall slot 51. Gas flow introduced into the
cavity electrode 5 through
supply gas ports 54 and 55 will flow out wall slot 51 and convect some of the
plasma species beyond wall
slot 51. Gases introduced into ports 55 may be like or different gases
compared to those introduced
through gas ports 54.
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Support structure 56 is made so that the cylinder electrode 50 can be
positioned toward or away
from wall slot 51 depending on the nature of the discharge desired. As an
alternative, cylinder electrode
50 may be positioned toward gas ports 54 allowing a discharge to be initiated
within the annular gap 53
near gas ports 54. A secondary gas may then be introduced through gas ports 55
to influence the plasma
energy level and number of metastable states present.
A third electrode treater drum 9 is positioned external and parallel to cavity
electrode 5 with its
outer diameter close to the wall slot 51. A film or web substrate 2 is
positioned using guide rollers 7 on
the outer diameter of treater drum 9. Substrate 2 is translated past wall slot
51 by the rotation of treater
drum 9 and exposed to plasma species converted through wall slot 51. Treater
drum 9 is positioned
outside the cavity electrode 5 in a curved recess 57 machined in the wall of
the cavity electrode 5 having
wall slot 51. Curved recess 57 is machined with a radius slightly greater than
the radius of treater drum
9, so that a small uniform clearance gap 58 exists between treater drum 9 and
cavity electrode 5. The
working or discharge gas is allowed to escape along gap 58 for continued
exposure of substrate 2.
Cavity electrode 5 and cylinder electrode 50 can be excited with either the
asymmetric pulse
voltage or the resonant RF signal. The treater drum 9 can be electrically
biased to a potential different
from that of the cavity electrode 5 or cylinder electrode 50 to aid in surface
treatment of substrate 2. This
bias can be in the form of an asymmetric pulse voltage or an RF signal of a
voltage amplitude less than
those of electrodes 5 and 50.
The end openings of the bore in cavity electrode 5 are machined with contoured
edges 59, so as to
provide a smooth transition of the electric-field gradients. The cavity ends
are blocked to axial gas flow
by the dielectric support structure 56 and shaft seals 60 on the outer
diameter of cylinder electrode 50.
The cavity electrode 5 is machined with fluid passages 61 for the flow of a
temperature-regulated fluid.
A cavity electrode constructed of aluminum has been tested and found to
operate satisfactorily. A
suitable stainless steel or other metal compatible with the gases or vapors
used can also be used.
Cylinder electrode 50 is constructed similar to electrode 8 in Fig. I .
Electrode 50 is center bored
to accommodate a partially hollowed support shaft 62, bearings 63, and shaft
seals 64. The bore diameter
is machined sufficiently larger than the support shaft 62 to produce an
annular passage 65 between
support shaft 62 and cylinder electrode 50. A temperature-regulated fluid is
introduced through inlet 66
into passage 65 and exited through outlet 67. The ends of support shaft 62 are
held by external shaft
mounts 68. For high-power discharges, cylinder electrode 50 can be made to
rotate slowly within cavity
electrode 5 to distribute the thermal loading. A grooved pulley 69 is fitted
to the end of the cylinder
electrode 50 to allow an external drive (not shown) to rotate the electrode.
Depending on the desired operating conditions, cylinder electrode 50 can be
used with or without
a dielectric covering. If electrode 50 is used with a plasma- or flame-sprayed
dielectric coating, or
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without a dielectric coating, cylinder electrode 50 can be machined with
contoured ends similar to 59 to
produce a smooth transition of the applied electric field.
Treater drum 9 is also machined with contoured edges 70 to minimize strong
localized electric
fields and a hollow interior 71 so that a temperature-regulated fluid can be
circulated through the drum in
a fashion similar to that in cylinder electrode 50. A support shaft 72 and
bearings 73 support treater drum
9 and allow it to rotate freely with the translating of substrate 2. A set of
take-up blocks 74 holding
support shaft 72 are used to accurately position treater drum 9 within curved
recess 57. Support shaft 72
is machined with fluid inlet passage 75 and outlet passage 76 for the
circulation of a temperature-
reguiated fluid. Shaft seals 77 prevent leakage of the temperature-regulated
fluid. Guide rollers 7 can be
integrated into the support structure 56 using bearings 78.
Alternative variations of the embodiment illustrated in Fig. 5 can be made
without substantially
departing from the scope of the invention. For example, treater drum 9 can be
used as a cylinder
electrode similar to cylindrical electrode 8 in Fig. 1. A sleeve electrode
similar to electrode 4 can then be
positioned next to the treater drum 9. A substrate can then be pretreated
prior to or post-treated following
treatment by the remote plasma discharge. For grafting, a desirable
configuration is to pretreat a
substrate with direct plasma exposure followed by a grafting treatment using
remote plasma treatment. A
polymer surface is functionalized with radicals or oxygen species using the
direct plasma treatment and
subsequently exposed to a monomer and monomer fragments produced by a
discharge in the cylindrical
cavity. Rapid polymer grafting has be accomplished using this technique.
Fig. 6 shows a midpiane cross-sectional end view of the lower portion of a
cylindrical cavity
electrode assembly, according to an alternative embodiment of the present
invention. According to this
embodiment, the cavity electrode comprises two pieces electrically connected
by a shunt inductor, to
simplify fabrication and to allow the use of distributed shunt inductors.
N. Resonant RF Excitation Method
Figs. 7 and 8 illustrate circuit diagrams that model the components and plasma
involved in a
resonant RF discharge. Fig. 7 is a circuit diagram for the resonant RF
excitation method illustrating the
basic circuit components and a generalized impedance model of the plasma,
plasma sheath, and dielectric
barrier. Fig. 8 is a circuit diagram illustrating distributed inductances of
connecting leads and electrodes
and a simplified model for distributed capacitance and resistance of the
plasma discharge of two sleeve
electrodes positioned on the same cylinder electrode.
In Fig. 7, RF generator 80 can take the form of a fixed-frequency source or a
variable-frequency
source. The frequency range specified of 1 MHz to 82 MHz is to cover the
industrial bands: 13.56 MHz,
27.12 MHz, 40.68 MHz, and 81.36 MHz and a low-frequency range of 1 MHz to
13.56 MHz. Isolator 81
is an optional component that minimizes potential damage to RF source 80
caused by excessive reflected
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power that may occur with load mismatch. RF source 80 typically has an output
impedance of SO ohms,
whereas the plasma load may be a few hundred ohms. 1ZF transformer 82 converts
the unbalanced output
of isolator 81 or source 80 to a balanced output. A balanced output has equal
current and opposite
voltage potential when properly balanced on the transmission line leads. The
balanced output of
transformer 82 feeds a balanced pi-matching network 83 comprising variable
inductors L1 and L2 and
variable capacitors C 1 and C2. By tuning the components of network 83, the
high impedance of the
parallel combination of shunt variable inductor 84 and plasma network 85 can
be matched to the RF-
source impedance. Inductor 84 is tuned so that the effective capacitance CP of
the plasma network 85
forms a parallel resonant circuit with a radian resonant frequency cy given by
cy(LSCp)~'' . This
frequency is tuned to the frequency of source 80, or the source frequency is
tuned to match c.~" once a
plasma is ignited. When these frequencies are matched, the impedance of the
parallel combination of CP
and LS will be predominately a real impedance for many gas mixtures over the
pressure range (about 10
Ton s P s about 1 Atmosphere). The resonant impedance is effectively matched
to the impedance of
source 80 using pi-matching network 83. The real impedance also allows a high
voltage 2Vd to be
generated across the plasma load for gas breakdown. Although the balanced
network is preferred, an
unbalanced network can also be used provided ground paths are such that shunt
inductor LS 84 and
plasma network 85 remain a resonant structure.
The Q or quality factor of the resonant circuit as measured by the ratio of
the resonant circuit
current Ir to the output current Io of the pi-matching network 83 is typically
3 to S. The increased current
flowing in shunt inductor 84 and plasma network 85 adds stability to the
plasma discharge.
The plasma is modeled as having a plasma sheath on each electrode represented
by a capacitance
C,h in parallel with resistance Rx," plasma body Zp, and capacitance Cd (86)
for a dielectric barrier. If a
dielectric barrier is not used on either electrode face, capacitance Cd is not
present and a d.c. or low-
frequency (w«cy) pulsed bias can be applied to the plasma discharge using
power supply 87. Inductors
L,e, and L,ez act as RF blocks for power supply 87, and capacitor Ce2 acts as
a d.c. block so that supply 87
is not shorted by shunt inductor 84.
For large electrode structures or high-frequency operation, wavelength effects
become important
and care must be observed to excite the electrode structure so that a uniform
voltage distribution occurs
along the electrode length. Fig. 8 illustrates a circuit model for the
electrode leads, electrode structure,
and plasma discharge for two sleeve electrodes positioned along the same
cylinder electrode. The plasma
impedance for sleeve electrode A is modeled as six series combinations R, and
C, and, for sleeve
electrode B, as six series combination RZ and CZ . Six variable inductors L~
(n=1,2,3,4,5,6) are connected
between electrodes A and B, so that the total inductance of the six inductors
forms a resonant circuit with
the series addition of the effective gap capacitance of electrodes A and B. A
network of connecting leads
of approximately equal length feeds the shunt inductors paralleled by the
distributed resistance and
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capacitance of the two plasma discharges. Connections +Vd and -Vd are
connected to the output leads of
a pi-matching network 83. Inductors L,, can be tuned independently to produce
a more uniform voltage
distribution along the length of electrodes A and B. The specific choice of
six parallel combinations is
used for illustration. In practice, the number of distributed inductive
elements will depend on the length
of electrodes A and B and the desired frequency of operation.
Experimental Data and Treatment Conditions
The excitation methods outlined in this specification each generate plasma
discharges with
characteristics distinct from conventional low-frequency (f<_60 kHz)
sinusoidal excitation methods.
Using an RCA P28A photo multiplier (PM) tube, the optical emission of three
different plasma
discharges were compared.
Fig. 9 is an oscillograph trace of the voltage waveform for an applied high-
voltage low-frequency
(f=1 kHz) sinusoidal signal and the PM response due to the resulting plasma
discharge. From the PM
response, it becomes clear that the plasma discharge turns off and on twice
per voltage cycle in a
sequence of multiple bursts. This plasma appears as a uniform "glow" to the
naked eye, but is not an
ignited or self sustained discharge. The duty cycle of the discharge, as
determined by the time a light
emitting plasma is present to the period of a voltage cycle, is typically only
a few percent.
Fig. 10 is an oscillograph trace of an asymmetric voltage pulse and the PM
response due to the
resulting plasma discharge. Since this discharge is capacitively coupled due
to the presence of a
dielectric barrier, current will only flow to the discharge with a change in
the applied voltage. The
voltage pulse {upper trace) has a rise time of approximately 1 microsecond and
reaches a peak amplitude
of close to 8 kilovolts. From the PM response (lower trace), a plasma is not
initiated until the voltage
pulse reaches an amplitude of around 4 kilovolts. The discharge ignites as a
whole and continues until
the voltage pulse peaks. During the plateau of the voltage pulse, the PM
response shows the decay time
of the plasma discharge (~--0.5 microseconds). The fast rise time of this
discharge technique allows for
higher power densities than those obtained with low-frequency sinusoidal
excitation. The discharge duty
cycle is typically 10%. The smaller secondary discharge occurs when a negative-
voltage pulse occurs
following the decay of the primary voltage pulse. This secondary voltage pulse
is due to the "kick-back"
voltage of the step-up pulse transformer used in the excitation circuit.
Fig. i 1 is an oscillograph trace of an applied high-frequency 13.56 MHz RF
voltage (upper trace)
applied to a resonant discharge and the PM response from the discharge (lower
trace). The PM response
clearly shows the plasma to be continuous since no flat regions exist in the
Vace. The 2a1 response of the
PM and second-harmonic content in the applied voltage are due to the nonlinear
behavior of the plasma
sheaths. Since this form of discharge is continuous, duty cycles up to 100%
can be obtained.
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Fig. 12 illustrates the scaling of power density with applied voltage for the
cylinder-sleeve
electrode geometry of Fig. 2 when excited as a resonant RF discharge. The
power density scales as the
applied voltage squared over a broad pressure range (76sPs760 Torr),
indicating that the discharge mode
is the same over this pressure range using the same discharge gas mixture.
The high-pressure plasma apparatuses and methods of the present invention have
been found
useful in achieving desired properties when used in conjunction with certain
plasma chemistries. Some
of these desired properties can be categorized as follows: surface cleaning,
enhanced wettability,
hydrophilicity/lipophilicity, surface grafting, and surface functionalization.
Surface cleaning refers to the removal of extraneous material from the surface
of a substrate, such
as a film, while leaving the substrate intact. Films and the like have oils,
surfactants, lubricants, etc., that
have been deposited on the surface during film formation or which bloom to the
surface from the film
body. 'These materials should be removed in order to adhere additional
materials to the film surface, such
as inks during printing of the surface or lamination of additional film or
other material.
Enhanced wettability (e.g., receding contact angle) refers to making a
substrate hydrophilic to a
high degree, where the treated substrate retains its hydrophilic state over an
extended period of time.
While other methods (e.g., corona treatment) provide hydrophilicity to
substances, these methods do not
cause the surface to retain this state over time (e.g., polyethylene films
that were corona treated will lose
hydrophilic properties normally within one day, while the present invention
will provide hydrophilicity
for several months). This permits the substrate to be used for this properly
after storage and the like. For
example, this properly provides films that (a) would have enhanced anti-fog
properties in food packaging
or (b) anti-static properties in films used to package electronics, etc. In
certain instances, prior processes
can achieve the extended wettability, but the present invention provides a
mode of achieving this property
in a commercially feasible manner that is both fast and reliable.
Similarly, hydrophobicity and lipophobicity can be achieved where the present
invention is used
with certain plasma chemistry. These properties relate to a substrate having
surfaces that can act as a
chemical barrier to a wide range of materials. It can provide a barrier to
hydrocarbons and water and
enhance slip, abrasion resistance, etc. (similar to Teflon coatings). Although
prior plasma treatments of
low pressure may achieve such properties, they were of academic nature or they
required extended
treatment while the present invention does so in fractions of a second so as
to be feasible on a
commercial, continuous line process.
When using the electrode configuration of Fig. S where the substrate may be
outside the electric
field, certain chemistries have been found to provide enhanced surface
grafting to achieve either
hydrophilic or hydrophobic properties without unwanted side reactions
occurring on the substrate
surface.
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21
The present invention also provides a means of achieving enhanced
functionality to the treated
surface. For example, where a nitrogen-containing gas and/or oxygen-containing
gas is used, one
achieves a highly functionalized surface of nitrogen-carbon and/or oxygen-
carbon species therein, while
only low functionality is achieved by convcntional modes of plasma treatment.
Treatment at moderate to
high pressures enables faster, more uniform, and increased functionalization
of a polymer surface relative
to low-pressure plasma and corona treatment, as shown in the following table.
Higher functionalization
usually means better wettability and adhesion for increased performance in
printing, lamination, dyeing,
etc. Remote low-pressure nitrogen plasma leads to very high functionalization
on stationary substrates.
A variety of gases can be used. While NZO/COZ works well, one may use other
systems, such as air,
nitrogen, oxygen, argon, CO~/OZ, and the like.
Eztent N
of to
Surface C
Functionalization Ratio
as
Measured
by
Surface
O+
l
Polymer4Treatert PressGas J/cm'CAH=o %C %O %N j0Nl
ton d ) C
PE HP-RF 76 N O, CO , 21 52 73 21 4.4 0.34
He
PE HP-Remote 89 N O, CO , 27 64 81 17 2.0 0.23
He
PE comna 760 Air 20 72 85 i 0.6 0.18
S
PE LPP 0.3 N,O, CO 0.03 68 81 14 0.8 0.18
PE has
[O+N]/C
less
than
0.01
HP =
hi
ressure
20-800
torr
, LPP
= low
ressure
lasma
<1
torr
Several areas of potential utility in food packaging applications have been
identified for plasma-
treated polymers. Polymer films plasma treated using high-pressure (800ZPZ50
Torr) discharge
technology according to the present invention were investigated for a variety
of applications.
Applications of particular interest include:
(1) The use of a high-power-density RF discharge to remove slip agents from
polymer films to allow
ink adhesion for printing;
(2) Polymer films with surfaces activated using a high-pressure plasma
discharge that are capable of
being pressure laminated without the use of an adhesive coating;
(3) The use of a high-pressure pulsed plasma discharge to produce film
surfaces with higher levels of
oxygen and nitrogen than can be obtained using conventional corona treatment
technology;
(4) Polymer films with fluorinated surfaces having increased hydrophobic
properties;
(5) Polymer filins having low advancing contact angle produced by grafting
acrylic acid to the
polymers surface;
(6) Polymer films treated with silane gases having potential anti-fog
applications; and
(7) Treatment of web substrates.
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22
These different applications are discussed in the following sections.
(1) Plasma Surface Cleaning for Printing Applications
The present invention was used to clean surface additives from polymer films.
The results of this
study are presented in Table 1. A polymer f lm with a heavy coating of a slip
agent was plasma treated
using a dielectric barrier cylinder - aluminum sleeve electrode geometry,
based on the cylinder-sleeve
electrode assembly of Fig. 2, excited with a resonant 1ZF discharge at 13.56
MHz. The high power density
of this discharge method is well suited for surface etching. Plasma discharges
using a CF; OZ mixture at
reduced pressure (P~50 Torr), and a He-CF,-02 discharge at atmospheric
pressure were used. These
studies quickly revealed that a threshold level of oxygen should be used for
sufficient removal of the slip
agent. The OZ concentration should be approximately 25% of the CF,
concentration or greater for
sufficient print adhesion. Greater oxygen concentrations should be avoided,
not only due to the potential
for combustion, but also due to the beta chain scission process that occurs on
polymers such as
polypropylene with increasing surface oxygen content in the presence of UV
radiation.
Any metal-faced electrode should not be constructed of a metal such as copper
that causes the
rapid recombination of atomic fluorine. The aluminum-oxide layer that quickly
forms on an aluminum-
faced electrode is relatively immune to this process provided chlorine is not
present.
A second set of experiments was perfona~ed using a CF4 Oz discharge at reduced
pressure to
remove glycerine- and silicone-based additives from a polyoiefin film. Corona
treatment is only partially
successful at removing silicone-based additives and is especially inadequate
with glycerine-based
additives. For these tests, a resonant RF discharge at reduced pressure
operation with the cylinder-sleeve
electrode geometry of Fig. 2 was used. The treatment data and results for tape
peel tests are listed in
Table 2.
In particular, Table 2 lists plasma-treatment conditions used to remove
surface additives from two
polyoIefm-based films using direct plasma exposure with a resonant 1tF
discharge at 13.56 MHz and a
tetrafluoromethane-oxygen plasma. ItF plasma treatment was performed using the
cylinder-sleeve
electrode geometry. A solvent-based ink was applied fve to ten minutes after
plasma exposure using a
nip-roller applicator. The ink was allowed to dry for approximately 30
minutes, and a tape peel test was
performed. 3M Scotch brand adhesive tape was firmly pressed onto a printed
sample, and then pulled
from the sample at 90 degrces to the surface with the level of ink removal
observed. The conditions
listed yielded essentially 100% ink adhesion with negligible ink removal,
whereas untreated samples
typically exhibited 80% or better ink removal with a tape peel test. For the
plasma dosages used, all
treated samples exhibited excellent print adhesion.
Despite the above results, treatment costs may be prohibitive to commercial
feasibility without
additional improvements. Tetrafluoromethane CF4 is an expensive specialty gas
commonly used in
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23
semiconductor fabrication for the etching of silicon oxide. The gas used was
purchased in a 220 ft'
cylinder at a cost of ~24ø/liter. The nominal flow rate of 1 liter/minute used
for a 15-cm-wide treatment
zone and Line speed of --10 m/minutes yields a treatment cost of ~16ø/m~ for
the CF, alone. With
treatment optimization, this cost could probably be cut in half, and with gas
re-circulation cut in half
again. This still yields a CF; gas cost of ~4ø/m2. As an alternative, sulfur
hexafluoride SF6 should be
considered. This gas is approximately 1/5 the cost of CF, due to its usage as
a dielectric insulator for
high-voltage applications. At reduced pressure operation Ps 100 Torr, SF6 can
be used with the resonant
RF discharge. Chlorine is another alternative, however the toxic and corrosive
aspects of chlorine would
need to be addressed.
When using R.F direct plasma at 50 to 760 Torr with a CF,/OZ plasma where the
OZ to CF, ratio is
greater than 0.2 (by volume), one achieves sufficient surface cleaning for
removal of low molecular
weight additives to cause promotion of adequate adhesion of a coating. In
comparison, corona discharge
in air and low-pressure plasma with CF,/OZ did not clean the surface
adequately enough to promote good
adhesion. Further, corona-treated surfaces left a sticky residue due to
inadequate ablation (volatilization),
while the RF direct plasma operated at SO to 760 Ton provided good ablation
and did not impede the
desired, subsequent re-blooming of the low molecular weight additives. In
order to obtain certain surface
propcrties (e.g., slip, anti-stick, anti-fog, etc.) on polymer surfaces, low
molecular weight additives are
blended into the polymer prior to extrusion so that they will bloom to the
surface after extrusion. These
low molecular weight species make it very difficult to get good adhesion to
the surfaces of a film.
Effcient surface cleaning at low temperatures (<60 C) prior to coating,
laminating, printing, etc.
processes operating at process speeds (>50 ft/min) leads to acceptable
adhesion without detrimental
damage to the underlying polymer surface.
The use of plasmas for surface cleaning is known. CF,/OZ plasmas are well
known for their
etching capabilities and are commonly used for cleaning surfaces in the
microelectronics industry. These
require extended treatment times and are not polymeric substances, which are
sensitive to high
temperatures.
The present invention provides advantages over the prior art. Only high
pressure (>20 Torr)
plasma operating in continuous mode with 1tF power has shown suffcient surface
cleaning to promote
adequate adhesion of a material to a polymer containing high amounts of
additives without damage to the
polymer.
CF, is a source of fluorine radicals and O~ is a source of oxygen radicals
that work together
synergistically to functionalize and volatilize organic species. Other
fluorine radical sources include SF6,
NF3 and Fi, etc. Oxygen radical sources include COZ, NO, SOZ. The unique
cleaning capabilities of the
RF direct plasma at 20-800 torn may be useful in the removal of organics in
circuit board manufacture.
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24
(2) Lamination of Plasma-Treated Polyolefins
The present invention was also used to investigate the potential for
laminating different polymer
surfaces that had undergone plasma exposure. Part of this work stemmed from
the concept of treating
one surface to produce predominantly acid functionality and treating a second
surface for predominantly
basic functionality, and then uniting the surfaces under pressure and heat to
produce covalent bonding. A
second concept was to deposit a fluoropolymer layer that would be conducive to
lamination. Lamination
between the following polymer pairs was attempted: PET-PE, EVA-PET, and EVA-
EVA. The treatment
conditions used in this study are listed in Table 3. Lamination was
accomplished by initially treating two
samples simultaneously and winding the samples on a common reel during plasma
treatment with treated
surfaces facing. Following plasma treatment, sections from the reel of treated
samples were hand fed into
a heated set of nip rollers. Several of these samples exhibited cohesive
failure dry, but no samples
exhibited significant adhesion after a water soaking. The poor results with
PET were most likely due to
the formation of a weak boundary layer on the PET surface caused by polymer
chain scission.
A set of lamination tests was performed on a polyethylene film with varying
levels of ethylene
vinyl acetate. PE film with EVA levels of 0%, 3%, 9%, and I 8% were treated
using the cylinder-sleeve
electrode geometry of Fig. 2 with a resonant RF discharge at reduced pressure.
Treatment conditions and
peel strengths are listed in Table 4. Treatment was performed with two film
strips treated simultaneously
and then wound on a common reel. The distance from the treatment zone to where
the two strips merged
was ~6 inches. Samples were laminated by using a manual hot press maintained
at ~170~'F. All samples
exhibited good dry adhesion, but only some of the short-exposure samples
exhibited adhesion after water
soaking.
In particular, Table 4 lists plasma-treatment conditions and peel strengths
for polyethylene film
that was plasma treated and then heat/pressure laminated. The polyethylene
film had varying degrees of
ethylene vinyl acetate on the plasma exposed side. Treatment was accomplished
using two cyIinder-
sleeve electrode assemblies with two supply reels of like film. The electrodes
were arranged so that just
after exiting the treatment zones, the two film strips merged with treatment
sides facing. Samples of the
resulting single strip were later pressure laminated at approximately I50
degrees centigrade. Laminated
samples were subjected to peel tests with the samples dry, or after water
soaking.
Fig. 13 shows a midplane cross-sectional end view of a twin-drum plasma
treater for laminating
films, according to one embodiment of the present invention. According to this
embodiment, the plasma
treater has two cylinder-sleeve electrode assemblies similar to that of Fig.
5, except that each sleeve
electrode comprises two electrodes electrically connected by a shunt inductor.
As shown in Fig. 13, two
films (A and B) enter the enclosure through a single roller-seal assembly, are
surface treated at different
cylinder-sleeve electrode assemblies, and are laminated together with their
treated surfaces facing each
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other by rollers of the roller-seal assembly, with the resulting laminated
product exiting the enclosure
through the roller-seal assembly.
Lamination capable of withstanding a water soaking is enabled by the formation
of covalent
bonds or significant van der Waals force bonding. The formation of covalent
bonds during the
lamination of relatively inert polymers such as polyethylene can be achieved
if the plasma treatment
process either forms radicals on the polymer surface or deposits an adhesive
boundary layer through
plasma polymerization or a grafting process. The examples presented in Table 5
used nitrogen plus 1%
nitric oxide as the working gas. This form of treatment produces surface
radicals that result in covalent
bonding. The strong dependence of treatment dosage on the wet pull strength
suggests the formation of a
low-molecular-weight boundary layer due to chain scission, or the presence of
few surface radicals to
produce covalent bonding. Polyethylene is fairly robust to W-induced chain
scission with low levels of
oxygen present. However, the formation of nitrogen dioxide NO2, due primarily
to the reaction of ozone
with nitric oxide, is a significant problem. Nitrogen dioxide is a strong
scavenger of radicals, as are
certain polymer additives. Higher-order oxides of nitrogen are probably
equally effective as radical
scavengers.
Below are listed several reactions and the corresponding rate constants for
the formation of
nitrogen dioxide and higher-order oxides:
Reaction k (cm'L$gg,~
1. N+O, - NO+OZ 5.7X lU"
2. NO + 03 - NOZ + OZ 1.8 x 10'"
3. NOZ + O3 ~ N03 + 3.2 x 10'"
OZ
4. NO + N03 - 2N20 2.6 x 10'"
5. NOZ + O - NO + OZ 1.7 x 10'" exp(-300
6. NOZ + rlO, - N=Os 3.8 x 10''z
7. NiOs + O -. 2NOZ 1 x 10'"
+ OZ
Reactions 2 and 3 are significant because ozone tends to be long lived and
with ~1% nitric oxide present,
a significant amount of nitrogen dioxide will be formed despite Reaction 1.
Fig. 14 illustrates the lifetimes of some various reaction products and ion
species for a micro
discharge modeled in air. Based on this graph, the lifetime of nitrogen
dioxide is suffcient to affect a
polymer surface external to the treatment zone. If oxygen of even a few
percent is present within the
discharge chamber, ozone levels will rise, as will nitrogen dioxide levels. A
slow treatment will subject a
treated surface to increased exposure to the nitrogen dioxide and higher-order
oxides. This exposure will
reduce the radical concentration on the surface of the treated polymer,
degrading lamination peel
strength.
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Using moderate to high pressures enables fast, uniform deposition of
fluorocarbons species onto a
polymer surface with reduced capital cost relative to low-pressure plasmas,
for applications such as
chemical barriers. The present invention provides means for fast deposition at
high processing speeds.
(3) Comparison of Pulse Discharge and Corona Discharge Surface Treatments
A study was made to compare surface treatments produced using atmospheric
pressure discharge
generated by pulsed-voltage excitation with conventional air-fed corona
discharge technology using a
small Enercon treater which could be used for film treatment. This treater
uses ceramic-coated bar-type
electrodes excited by a variable frequency (6 kHz to 30 kHz), half sine-wave
signal.
The atmospheric-pressure dielectric-barrier discharge was excited by applying
a fast, asymmetric
voltage pulse to the cylinder-sleeve electrode geometry of Fig. 2. A pulse-
repetition frequency of 10 kHz
was typically used. Several different gas mixtures were used to treat linear
low-density polyethylene and
polypropylene. Treated samples were analyzed using ESCA surface analysis to
determine elemental
surface concentration of oxygen and nitrogen. The scaling of surface oxygen
and nitrogen for the pulsed
atmospheric discharge and the Enercon corona-treated surfaces are shown in
Fig. 15 and Fig. 16.
In particular, Fig. 15 and Fig. 16 illustrate the scaling of surface oxygen
and nitrogen with
treatment dosage on polypropylene and polyethylene films. Treatments with
various gases at
atmospheric pressure using the electrode geometry in Fig. 2 with asymmetric
pulsed voltage excitation
are compared to treatments using a commercial corona treater 'CDT' in air. The
surface concentrations
of oxygen and nitrogen were measured using X-ray photoelectron spectroscopy
(XPS) surface analysis.
Treatment dosage is the discharge power density times the plasma exposure
time. These data illustrate
that using air, carbon dioxide + oxygen, or nitrogen + 1% nitric oxide, the
pulsed dielectric-barrier
discharge is capable of producing high concentrations of oxygen on both
polymers.
The gas mixture of nitrogen + 1 % nitric oxide produced significant levels of
surface nitrogen on
both polypropylene and polyethylene.
(4) Surface Fluorination
Surface fluorination was investigated as a means to increase hydrophobic
properties of polymer
surfaces and potentially decrease surface friction. Film samples of
polypropylene, polyethylene, and PET
were first treated in a commercial low-pressure (Ps 1 Torr) system with a gas
mixture of argon and
perfluoromethane CZF6 for two minutes. Advancing contact angles of 125 degrees
for water were
obtained for ail three polymers.
Several attempts were made using the present invention to fluorinate film
surfaces using a pulsed
discharge and an RF discharge at atmospheric pressure with a gas mixture of
helium and
tetrafluoromethane CF,. These experiments yielded contact angles of 104 to 109
degrees. It was
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27
subsequently learned that copper has a high probability for recombination of
atomic fluorine, and copper
was the material of the upper electrode. Also the presence of only trace
amounts of oxygen were
sufficient to cause the formation of a hydrophilic surface. Changing to a
stainless-steel electrode and
increasing the gas flow to offset oxygen infiltration yielded improved
results. Table 5 lists treatment
conditions and contact angles for the treatment of polyethylene using He + CF,
and He + CZF6 mixtures.
(S~ Polymer Grafting of Acrylic Acid
Polymer grafting using various monomers has been used to produce extremely
hydrophilic
surfaces. Grafting is normally initiated by the presence of peroxides or
peroxy radicals that have been
generated on the polymer surface via plasma, W, or gamma-ray exposure with the
monomer applied in
a gas or liquid phase. These treatment methods have been effective at
producing hydrophilic surfaces,
however treatment times are typically measured in minutes.
In an attempt to grab the monomer acrylic acid with short exposure times, a
two-step process was
configured. A polymer filin was first exposed to direct plasma treatment,
using the cylinder-sleeve
electrode assembly of Fig. 2, to functionalize the surface with oxygen species
and radicals. The film was
then exposed to plasma species, monomer, and monomer fragments generated by
the remote plasma
geometry of Fig. 5. Table 6 lists treatment conditions and surface properties
for the treatment of
polyethylene using two different gas mixtures.
In particular, Table 6 lists plasma treatment conditions used to treat a
linear low-density
polyethylene film by grafting the monomer acrylic acid onto a pretreated
surface. Surface pretreatment
was performed using the cylinder-sleeve electrode geometry with a 45 kHz
negative-voltage pulse applied
to the sleeve electrode. Grafting of acrylic acid was performed by passing a
supply gas over the
monomer and then introducing the supply gas plus monomer vapor into the cavity
of the cylindrical
cavity electrode assembly. A resonant 1tF plasma was excited at 13.56 NIHz,
and the polymer film
translated past the cavity slot. The 10-second exposure sample using a sulfur
dioxide-oxygen supply gas,
was slightly tacky but did not "block" on the wind-up spool. These examples
illustrate the technique's
ability to graft the monomer acrylic acid using short exposure times.
According to these data, treatment
times as short as 2 and 5 seconds produced hydrophilic surfaces with good anti-
fog properties.
For organo-sulfur surfaces,1tF remote plasma at 50 to 760 torn operated with
acrylic acid and SOZ
in the plasma leads to a highly wettabie, high-tack surface for applications
such as adhesion promotion
and anti-fog. Plasma deposition is known in the low-pressure regime for
species such as acrylic acid but
requires several minutes of treatment or special monomer delivery systems. The
present invention
enables higher speeds, thereby providing a commercially feasible treatment of
surfaces. The present
invention also provides better control of chemistry due to the reaction being
out of the electric field.
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(6) Silane Treatments for Anti-Fog Applications
The different silanes have been used for many years by the semiconductor
industry to deposit
amorphous-silicon and silicon-dioxide layers for dielectric barriers using
plasma deposition. The silicon-
dioxide-type layers tend to be very hydrophilic and have recently been applied
to polymer surfaces.
Table 7 and Table 8 list treatment conditions and hydrophilic properties for
the treatment of polyethylene
film with silicon tetrachloride and dichlorosilane. With short treatment
times, very wettable surfaces can
be produced with the potential for anti-fog applications.
In particular, Table 7 lists plasma treatment conditions used to coat
polyethylene films with a
silicon-oxide layer using silicon tetrachloride with nitrous oxide and carbon
dioxide as oxidants. These
treatments yield a highly hydrophilic surface. From the elemental surface
analysis data, surface layers
close to silicon dioxide are produced with low levels of chlorine or nitrogen
incorporated. Table 8 lists
plasma treatment conditions used for treating polyethylene f lm with
dichlorosilane and carbon dioxide to
produce a silicon-oxide layer. This treatment produced very hydrophilic
surfaces similar to the
treatments in Table 7.
The hydride silane SiH" which is commonly used in the semiconductor industry,
yields fast
deposition rates when used with a small amount of an oxidizer. This silane is
pyrophoric and requires
special handling. The chlorine-containing silanes, dichlorosilane (SiCIZH2)
and silicon tetrachloride
{SiCI,), are less flammable but more toxic and corrosive. Both SiCI~HZ and
SiCI, react with air and
moisture to produce hydrogen chloride HCI. When used in a plasma discharge,
these silanes react
completely. Excess flow rates, oxidizer or power densities tend to produce
powder deposits rather than
film deposits.
For silica-like surfaces,1tF direct plasma at 50 to 760 Ton operated with a
silane and an oxidant
leads to a stable, highly wettable surface with low coefficient of friction
for applications such as adhesion
promotion and anti-fog. 1ZF direct plasma treatment at moderate to high
pressures will be cheaper than
the prior-art, low-pressure plasma treatment, and may have faster deposition
rates capable of being used
in processes having faster speeds. The present invention may also provide much
rougher surfaces having
lower coefficient of friction and better machinability. 1ZF direct plasma at
20-800 torn will put down a
more homogeneous coating and have greater deposition rates. It is likely that
any volatile silicon
(SiXYIIz, where X= CI, F, Br, and where y+~-4) with any volatile oxidant
(e.g., OZ, NO, NZO, NOZ, N20"
CO, CO~, SOZ) will be produce a silica-like surface under plasma. Adhesion,
deposition rate,
homogeneity; wettability and roughness will vary based on the particular
apparatus used.
(7) Treatment of Web Substrates
Fibrous web substrates, either woven or nonwoven, can also be effectively
treated using either
discharge technique. Nonwoven substrates made from the relatively inert
polymers of polypropylene or
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29
polyethylene are good candidates for the surface modification available using
the plasma discharge
methods disclosed. Spunbond polypropylene has widespread use in applications
requiring absorbency.
Topical surfactants have traditionally been used to render polypropylene webs
hydrophilic. Surfactants,
however, provide limited wettability in that they tend to be degraded or
removed with rinsing and cannot
withstand certain chemical environments. Environmental concerns are also
beginning to place
restrictions on the use of surfactants with this type of substrate since
nonwovens are often used with
disposable products. Chemical additives called melt additives, which are added
to a polymer base prior
to the melting and extrusion process, can also produce hydrophilic surfaces.
These products rely either
on the mobile or migratory properties of a polymer that allow the additive to
"bloom" to the surface with
time, or the rheological processes involved during extrusion that allow some
of the additive to remain on
the polymer surface. Relying on the migratory properties of the polymer tends
to produce a surface that
alters with time, and control of the extrusion process is diffcult.
Plasma surface treatment can produce durable surface treatments. Tables 9 and
10 list
performance data for a 20 gr/mi polypropylene, spunbond treated using the RF
resonant discharge at a
reduced pressure of ~55 Torr.
Table 9 illustrates the excellent performance of treatments using
dichlorosilane and silicon
tetrachloride with small amounts of an oxidizer present. The wetting
performance is listed by the number
of water droplets absorbed at 0.1 second, 0.5 second, and 5 seconds out of ten
droplets placed randomly
on a sample. Only after a rinsing in distilled water did the performance of
the dichiorosilane-treated
samples degrade. All droplets were, however, still absorbed within five
seconds. The silicon
tetrachloride samples had excellent wettability and could probably be treated
with an even lower dosage.
Table 10 lists performance data for the same 20 gr/m2 polypropylene, spunbond
treated using
sulfur dioxide, nitrogen, and an oxidant. Although these samples do not have
the rapid wettability
performance of the silane-treated samples, they still exhibit rewetting
following a vigorous rinsing in
warm (--~43 °C) water. Some yellowing and odor were present on the
treated samples. Water rinsing
removed any residue and the discoloration.
Successful plasma treatment of polypropylene web substrates relies in part on
controlling the beta
chain scission process that occurs with polypropylene due to the presence of
ultraviolet radiation and
using a surface functionalization that is resistant to migration. Silicon
tetrachloride and dichlorosilane
are highly reactive and appear to produce highly polar groupings or matrices
on the polymer surface that
are resistant to migration. The sulfur dioxide treatments seem to produce
large functionalities such as
SOOOH groups which also tend to be resistant to migration due to their size.
Some degree of polymer
cross linking is also occurring with plasma exposure. This reduces surface
migration of the polar groups
by fixing or chaining the surface polymer matrix.
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Comparison of Discharge Technologies
As described in earlier sections, high-pressure plasma treatment of film
substrates uses one or
more electrode structures excited by either a high-voltage asymmetric pulsed
voltage or a high-frequency
resonant RF signal. The electrode designs are capable of generating uniform
well-defined, high-power-
density plasma discharges over the pressure range from about 10 Ton to about 1
Atmosphere. The
asymmetric voltage excitation can be used with essentially any gas
at.atmospheric pressure. For the
resonant RF excitation, a buffer gas, typically helium, and a few to several
percent of a working gas are
used to operate at atmospheric pressure. Reduced-pressure operation, however,
allows the resonant RF
discharge to be generated with a wide variety of gas mixtures. Reduced-
pressure operation also allows
significant savings with the use of specialty gases. Both excitation methods
can produce power densities
greater than the low-frequency (fs30 kHz) sinusoidal excitation used in
conventional corona treaters.
Table I 1 summarizes the operating parameters of the discharge techniques of
the present
invention as compared to conventional corona-discharge technology. Due in part
to the high duty cycle
obtained with the resonant RF discharge technique, power densities ten to a
hundred times higher than
pulse discharges or corona discharges can be obtained. At atmospheric-pressure
operation, this method
requires heavy use of an inert gas such as argon or helium. Reducing the
operating pressure to 150 Torr
or less allows the discharge technique to be used with essentially any gas
mixture. The pulse discharge
and corona discharge operate at atmospheric pressure with essentially any gas,
but with lower power
densities. The pulse discharge produces plasma power densities higher than the
corona discharge, and
allows some control over which plasma species bombard the substrate.
It will be further understood that various changes in the details, materials,
and arrangements of
the parts which have been described and illustrated in order to explain the
nature of this invention may be
made by those skilled in the art without departing from the principle and
scope of the invention as
expressed in the following claims.
SUBSTITUTE SHEET ( rule 26 )
CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/1
N ~ N N Z Z N Z Z (/7 N Z
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CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/ 2
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CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/3
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CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/ 4
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CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/ 5
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CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/ 6
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CA 02295729 1999-12-30
WO 99/04411 PCT/IB98/01058
30/ 7
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CA 02295729 1999-12-30
WO 99/04411 PCT/1B98/01058
30/ 8
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CA 02295729 1999-12-30
WO 99104411 PCT/IB98/01058
30/9
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CA 02295729 1999-12-30
WO 99104411 PCT/IB98/01058
30/10
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SUBSTITUTE SHEET ( ruie 26 )
