MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE DISCHARGES *...
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Transcript of MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE DISCHARGES *...
MULTISCALE SIMULATION OF FUNCTIONALIZATION OF SURFACES USING ATMOSPHERIC PRESSURE
DISCHARGES*
Ananth N. Bhoja) and Mark J. Kushnerb)
a)Department of Chemical and Biomolecular EngineeringUniversity of Illinois, Urbana, IL 61801 [email protected]
b)Department of Electrical and Computer EngineeringIowa 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.
Iowa State University
Optical and Discharge Physics
OUTLINE
ICOPS05_agenda
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
Iowa State University
Optical and Discharge Physics
FUNCTIONALIZATION OF SURFACES USING PLASMAS
ANANTH_ICOPS05_01
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
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Optical and Discharge Physics
SURFACE MODIFICATION OF POLYMERS
ANANTH_ICOPS05_02
Functionalization increases surface energy and enhances wettability and adhesion.
Akishev, et al, Plasmas Polym., 7, 261 (2002).
Pulsed atmospheric discharges (coronas) are widely used in industry to treat commodity polymers like polypropylene (PP).
Tantec, Inc.
PE Film
Iowa State University
Optical and Discharge Physics
TREATMENT OF NON-PLANAR SURFACES
ANANTH_ICOPS05_03
Penetration of plasma species into surface features determines the extent and uniformity of functionalization.
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.
M. Strobel, 3M
Untreated PP Plasma Treated PP
Iowa State University
Optical and Discharge Physics
SCHEMATIC OF THE 2-D MODELING PLATFORM
ANANTH_ICOPS05_04
Unstructured mesh
Plasma hydro-dynamics (multi-fluid, Poisson’s equation)
Radiation transport
Integrated plasma-surface kinetics model
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Optical and Discharge Physics
DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE
ANANTH_ICOPS05_05
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
MIN MAX
Iowa State UniversityOptical and Discharge Physics
PLASMA PROPERTIES IN He/O2/H2O MIXTURES
- 5 kV, 760 Torr He/O2/H2O=98/1/1, 0 – 2.5 ns
ANANTH_ICOPS05_06
Potential (V) Te (eV) [Positive Ions] cm- 3 [O] cm- 3
- 5000 – 0 0 – 7.5 1011 – 1014 1011 – 1014
ANIMATION SLIDE
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Optical and Discharge Physics
PLASMA PENETRATION INTO SURFACE FEATURES
ANANTH_ICOPS05_07
- 5 kV, 760 Torr, He/O2/H2O=98/1/1
[e] cm- 3
t = 2.5 ns t = 5 ns
t = 2.5 ns t = 5 ns
[O] cm- 3
t = 2.5 ns t = 5 ns
[Positive
ions] cm- 3
MIN MAX
1010 – 1013
1011 – 1013
1010 – 1013
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Optical and Discharge Physics
At higher O2 fraction, the density of reactive [O] radicals at the surface increases during the pulse.
ANANTH_ICOPS05_08
- 5 kV, 760 Torr
(log scale)
[O] cm- 3
EFFECT OF O2 FRACTION ON [O] DENSITY
He/O2/H2O = 98 / 1 / 1
He/O2/H2O = 69 / 30 / 1
t = 5 ns
2x1011t = 3 ns 2x1014
He/O2/H2O = 89 / 10 / 1
t = 3 ns
ANANTH_ICOPS05_09
Iowa State UniversityOptical and Discharge Physics
SURFACE REACTION MECHANISM FOR PP
Gas phase radicals abstract H from the surface and produce radical sites on the PP surface.
1R. Dorai and M.J.Kushner, J. Phys. D 36, 666 (2003).
Further reactions lead to formation of functional groups like peroxy, alkoxy, alcohols and other groups1.
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Optical and Discharge Physics
DENSITY OF SURFACE RADICALS
ANANTH_ICOPS05_10
- 5 kV, 760 Torr, 10 kHz, He/O2/H2O=98/1/1
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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Optical and Discharge Physics
DENSITY OF -OH FUNCTIONAL GROUPS
ANANTH_ICOPS05_11
- 5 kV, 760 Torr, He/O2/H2O=98/1/1, 10 kHz
Non-uniformities occur on the macroscopic and microscopic (< 1 µm) length scales; and are site dependent
Iowa State University
Optical and Discharge Physics
TEMPORAL VARIATION OF FLUXES WITH O2 FRACTION
ANANTH_ICOPS05_13
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 s at 10 kHz)
IP ENDIP START
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Optical and Discharge Physics
SPATIAL VARIATION OF FLUXES WITH O2 FRACTION
ANANTH_ICOPS05_13
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 s at 10 kHz)
[O2]=75% [O2]=10%
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Optical and Discharge Physics
EFFECT OF O2 FRACTION ON TREATMENT
ANANTH_ICOPS05_14
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
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Optical and Discharge Physics
HUMID AIR - EFFECT OF VOLTAGE POLARITY
ANANTH_ICOPS05_15
760 Torr, N2/O2/H2O=79/20/1 cm- 3
- 15 kV, 2.5 ns
MAX
Positive discharge spreads more uniformly on surface.
Surface recombination of O+ enhances [O] at surface.
1010 - 1013 +15 kV, 10 ns
- 15 kV, 2.5 ns
+15 kV, 10 ns
1011 - 1014
[O]
[e]
MIN
1011 - 1014 5x1011 - 5x1014
log scale
[e]
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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.
ANANTH_ICOPS05_16
760 Torr, N2/O2/H2O=79/20/1, 10 kHz, 0.15 s
ANANTH_ICOPS05_17
Iowa State UniversityOptical 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.
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Optical and Discharge Physics
NON-UNIFORMITY DUE TO ‘SHADOWING’
ANANTH_ICOPS05_18
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
[CROSSLINKS]
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Optical and Discharge Physics
CONCLUDING REMARKS
ANANTH_ICOPS05_19
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.