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



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



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



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



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



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



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



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


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.



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.



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



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



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%



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



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]



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


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.



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]



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.

multiscale simulation of functionalization of surfaces using atmospheric pressure discharges *

Documents

surface increases

discharge physicsschematic

surface featuresananth

surface energy

surface groups

roughness of surface

o alkoxy co peroxy co

o densityheo2h2o