MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE

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MULTISCALE SIMULATION OF FUNCTIONALIZATION OF
SURFACES USING ATMOSPHERIC PRESSURE
DISCHARGES Ananth N. Bhoja) and Mark J.
Kushnerb) a)Department of Chemical and
Biomolecular Engineering University of Illinois,
Urbana, IL 61801 bhoj_at_uiuc.edu b)Department of
Electrical and Computer Engineering Iowa State
University, Ames, IA 50011 mjk_at_iastate.edu http
//uigelz.ece.iastate.edu 32nd IEEE International
Conference on Plasma Science, 18th 23rd June
2005, Monterey, CA.
Work supported by the NSF.
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OUTLINE
? Functionalization of Surfaces using Plasmas ?
Modeling Platform ? Pulsed Coronas in Humid Air,
He/O2/H2O mixtures ? Uniformity of surface
treatment ? O2 fraction ?
Discharge Polarity ? Surface
microstructure ? Concluding remarks
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ICOPS05_agenda
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FUNCTIONALIZATION OF SURFACES USING PLASMAS
? Functionalization of surfaces occurs by their
interaction with plasma species - ions, radicals,
and photons. For hydrocarbon polymer
? Surface groups created in an O2 containing
plasma include ? Acids C(OH)O
? Carbonyl -CO ? Alkoxy C-O
? Peroxy C-O-O ? Alcohols
C-OH
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SURFACE MODIFICATION OF POLYMERS
? Pulsed atmospheric discharges (coronas) are
widely used in industry to treat commodity
polymers like polypropylene (PP).
? Tantec, Inc.
? Functionalization increases surface energy and
enhances wettability and adhesion.
? Akishev, et al, Plasmas Polym., 7, 261 (2002).
? PE Film
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TREATMENT OF NON-PLANAR SURFACES

• M. Strobel, 3M

? Roughness of surface to be treated may range
from 100s nm to few µm.

• SEM of polyester fabric (Borcia, et al, Plasma
  Sources Sci. Technol, 12, 235 (2003).

? Penetration of plasma species into surface
features determines the extent and uniformity of
functionalization.
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SCHEMATIC OF THE 2-D MODELING PLATFORM

• Unstructured mesh
• Plasma hydro-dynamics (multi-fluid, Poissons
  equation)
• Radiation transport
• Integrated plasma-surface kinetics model

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DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE

• Corona treating a polymer on the grounded
  electrode acts as a DBD.
• Gas mixtures
• ? He/O2/H2O
• ? Humid air - N2/O2/H2O
• How does uniformity of treatment vary over
  microscopic and macroscopic scales when
  repetitively pulsed?

? PRF 10 kHz
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PLASMA PROPERTIES IN He/O2/H2O MIXTURES
Potential (V)
Te (eV)
Positive Ions cm- 3
O cm- 3
- 5000 0 0 7.5
1011 1014 1011 1014
? - 5 kV, 760 Torr He/O2/H2O98/1/1, 0 2.5 ns
ANIMATION SLIDE
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MIN
MAX
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PLASMA PENETRATION INTO SURFACE FEATURES
e cm- 3
1010 1013
t 2.5 ns
t 5 ns
O cm- 3
1010 1013
t 2.5 ns
t 5 ns
Positive ions cm- 3

1011 1013
t 2.5 ns
t 5 ns
? - 5 kV, 760 Torr, He/O2/H2O98/1/1
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MAX
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EFFECT OF O2 FRACTION ON O DENSITY
He/O2/H2O 98 / 1 / 1
He/O2/H2O 89 / 10 / 1
t 5 ns
t 3 ns
? At higher O2 fraction, the density of reactive
O radicals at the surface increases during the
pulse.
He/O2/H2O 69 / 30 / 1

O cm- 3
2x1011
2x1014
t 3 ns
(log scale)
? - 5 kV, 760 Torr
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SURFACE REACTION MECHANISM FOR PP
? Gas phase radicals abstract H from the surface
and produce radical sites on the PP surface.
? Further reactions lead to formation of
functional groups like peroxy, alkoxy,
alcohols and other groups1.
1R. Dorai and M.J.Kushner, J. Phys. D 36, 666
(2003).
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DENSITY OF SURFACE RADICALS

• ALKYL (PP? ) are formed by
• O PP ? PP? OH
• Nonuniformities result from both fluxes and
  shadowing.

• ALKOXY (PP-O?)
• O3 PP? ? PP-O?
• O PP? ? PP-O?
• take longer to form
• but eventually
• dominate.

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? - 5 kV, 760 Torr, 10 kHz, He/O2/H2O98/1/1
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DENSITY OF -OH FUNCTIONAL GROUPS

• Non-uniformities occur on the macroscopic and
  microscopic (lt 1 µm) length scales and are site
  dependent

? - 5 kV, 760 Torr, He/O2/H2O98/1/1, 10 kHz
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TEMPORAL VARIATION OF FLUXES WITH O2 FRACTION
IP START
IP END

• Larger fraction of reactive O is lost in the
  interpulse period at higher O2 fraction by
  three-body recombination and ozone formation.
• Optimum composition is at O2 10 with
  significant O flux maintained through the long
  interpulse period.

• At position along surface 0.05 cm,
  He/O2/H2O99-x/x/1
• IP - interpulse period (duration 100 ms at 10
  kHz)

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SPATIAL VARIATION OF FLUXES WITH O2 FRACTION
O210
O275

• As O2 fraction increases, there is greater
  non-uniformity in fluxes of O and O3 at the
  surface.

• At end of IP, He/O2/H2O99-x/x/1
• IP - interpulse period (duration 100 ms at 10
  kHz)

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EFFECT OF O2 FRACTION ON TREATMENT

• Macroscopic non-uniformities in OH and OO
  ultimately arise from significant spatial
  variation of O, O3 and OH fluxes.
• Microscopic scale non-uniformities arise due to
  transport limitations of reactive O at higher
  O2 fractions.

? - 5 kV, 760 Torr, He/O2/H2O99-x/x/1, 10 kHz,
1 s
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HUMID AIR - EFFECT OF VOLTAGE POLARITY
- 15 kV, 2.5 ns
15 kV, 10 ns
? O
? e
? e
- 15 kV, 2.5 ns
1011 - 1014
1010 - 1013
15 kV, 10 ns

• Positive discharge spreads more uniformly on
  surface.
• Surface recombination of O enhances O at
  surface.

1011 - 1014
5x1011 - 5x1014
? 760 Torr, N2/O2/H2O79/20/1
cm- 3
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MIN
log scale
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EFFECT OF POLARITY ON TREATMENT

• Pulsed positive polarity operation gives higher
  uniformity of functionalization due to more
  uniform formation of O and O3 near the surface.

? 760 Torr, N2/O2/H2O79/20/1, 10 kHz, 0.15 s
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IONS AND PHOTONS IN SURFACE CHEMISTRY

• UV photons from the plasma can cleave surface
  bonds.
• Ions striking the surface transfer energy and can
  activate sites.
• Postulate creation of intermediate active sites.
• Active sites may crosslink in addition to
  standard reaction pathways.

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NON-UNIFORMITY DUE TO SHADOWING
CROSSLINKS

• Line-of-sight photon transport fails to reach
  deep inside surface features.
• Crosslinking is relatively higher on surface
  features exposed to the avalanche where ion and
  photon fluxes are higher.

? - 5 kV, 760 Torr, He/O2/H2O98/1/1, 10 kHz,
0.2 s
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CONCLUDING REMARKS
? Multiscale modeling using unstructured meshes
was applied to study DBD corona treatment of PP
surface with microstructure. ? Non-uniformity in
functionalization occurs on the macroscopic (few
mm) and microscopic ( lt 1 µm) scales. ? In
He/O2/H2O mixtures, the O2 fraction can be used
to optimize uniformity of treatment. ? In humid
air, pulsed positive polarity discharges produce
more uniform functionalization. ? Shadowing by
surface microstructure results in non-uniform
crosslinking with a mechanism using photon and
ion fluxes.
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