Transcript No Slide Title

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 [email protected]
b)Department
of Electrical and Computer Engineering
Iowa State University, Ames, IA 50011 [email protected]
http://uigelz.ece.iastate.edu
32nd IEEE International Conference on Plasma Science,
18th – 23rd June 2005, Monterey, CA.
*Work supported by the NSF.
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
ICOPS05_agenda
Iowa State University
Optical and Discharge Physics
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 -C=O
 Alkoxy –C-O
 Peroxy –C-O-O
 Alcohols –C-OH
ANANTH_ICOPS05_01
Iowa State University
Optical and Discharge Physics
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.
 PE Film
ANANTH_ICOPS05_02
 Akishev, et al, Plasmas Polym., 7, 261 (2002).
Iowa State University
Optical and Discharge Physics
TREATMENT OF NON-PLANAR SURFACES
Untreated PP
Plasma Treated PP
 M. Strobel, 3M
 SEM of polyester fabric
(Borcia, et al, Plasma Sources
Sci. Technol, 12, 235 (2003).
 Roughness of surface to be treated
may range from 100s nm to few µm.
 Penetration of plasma species into surface features determines
the extent and uniformity of functionalization.
ANANTH_ICOPS05_03
Iowa State University
Optical and Discharge Physics
SCHEMATIC OF THE 2-D MODELING PLATFORM
 Unstructured mesh
 Plasma hydrodynamics (multi-fluid,
Poisson’s equation)
 Radiation transport
 Integrated plasmasurface kinetics model
ANANTH_ICOPS05_04
Iowa State University
Optical and Discharge Physics
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
ANANTH_ICOPS05_05
Iowa State University
Optical and Discharge Physics
PLASMA PROPERTIES IN He/O2/H2O MIXTURES
Potential (V)
- 5000 – 0
Te (eV)
0 – 7.5
 - 5 kV, 760 Torr
He/O2/H2O=98/1/1, 0 – 2.5 ns
ANANTH_ICOPS05_06
MIN
[Positive Ions] cm- 3
1011 – 1014
ANIMATION SLIDE
MAX
[O] cm- 3
1011 – 1014
Iowa State University
Optical and Discharge Physics
PLASMA PENETRATION INTO SURFACE FEATURES
[e] cm- 3
1010 – 1013
t = 2.5 ns
t = 5 ns
[O] cm- 3
1010 – 1013
t = 5 ns
t = 2.5 ns
[Positive
ions] cm- 3
1011 – 1013
t = 2.5 ns
t = 5 ns
 - 5 kV, 760 Torr, He/O2/H2O=98/1/1
ANANTH_ICOPS05_07
MIN
MAX
Iowa State University
Optical and Discharge Physics
EFFECT OF O2 FRACTION ON [O] DENSITY
He/O2/H2O = 98 / 1 / 1
t = 5 ns
He/O2/H2O = 69 / 30 / 1
He/O2/H2O = 89 / 10 / 1
t = 3 ns
 At higher O2 fraction, the density of
reactive [O] radicals at the surface
increases during the pulse.
[O] cm- 3
t = 3 ns
 - 5 kV, 760 Torr
ANANTH_ICOPS05_08
2x1011
2x1014
(log scale)
Iowa State University
Optical and Discharge Physics
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).
ANANTH_ICOPS05_09
Iowa State University
Optical and Discharge Physics
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.
 - 5 kV, 760 Torr, 10 kHz, He/O2/H2O=98/1/1
ANANTH_ICOPS05_10
Iowa State University
Optical and Discharge Physics
DENSITY OF -OH FUNCTIONAL GROUPS
 Non-uniformities occur on the
macroscopic and microscopic
(< 1 µm) length scales; and are
site dependent
 - 5 kV, 760 Torr, He/O2/H2O=98/1/1, 10 kHz
ANANTH_ICOPS05_11
Iowa State University
Optical and Discharge Physics
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/H2O=99-x/x/1
 IP - interpulse period (duration – 100 ms at 10 kHz)
ANANTH_ICOPS05_13
Iowa State University
Optical and Discharge Physics
SPATIAL VARIATION OF FLUXES WITH O2 FRACTION
[O2]=10%
[O2]=75%
 As O2 fraction increases, there is greater non-uniformity in
fluxes of [O] and O3 at the surface.
 At end of IP, He/O2/H2O=99-x/x/1
 IP - interpulse period (duration – 100 ms at 10 kHz)
ANANTH_ICOPS05_13
Iowa State University
Optical and Discharge Physics
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/H2O=99-x/x/1, 10 kHz, 1 s
ANANTH_ICOPS05_14
Iowa State University
Optical and Discharge Physics
HUMID AIR - EFFECT OF VOLTAGE POLARITY
- 15 kV, 2.5 ns
 [e]
1011 - 1014
 [e]
5x1011 - 5x1014
cm- 3
MIN
ANANTH_ICOPS05_15
 [O]
+15 kV, 10 ns
log scale
1011 - 1014
- 15 kV, 2.5 ns
1010 - 1013
+15 kV, 10 ns
 Positive discharge spreads more
uniformly on surface.
 Surface recombination of O+
enhances [O] at surface.
 760 Torr, N2/O2/H2O=79/20/1
MAX
Iowa State University
Optical and Discharge Physics
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/H2O=79/20/1, 10 kHz, 0.15 s
ANANTH_ICOPS05_16
Iowa State University
Optical and Discharge Physics
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.
ANANTH_ICOPS05_17
Iowa State University
Optical and Discharge Physics
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/H2O=98/1/1, 10 kHz, 0.2 s
ANANTH_ICOPS05_18
Iowa State University
Optical and Discharge Physics
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 ( < 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.
ANANTH_ICOPS05_19
Iowa State University
Optical and Discharge Physics