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RADICAL GENERATION AND POLYMER SURFACE FUNCTIONALIZATION IN FLOWING ATMOSPHERIC
PRESSURE PULSED DISCHARGES*
Ananth N. Bhoja) and Mark J. Kushnerb)
a)Department of Chemical and Biomolecular Engineering,University of Illinois, Urbana, IL. [email protected]
b)Department of Electrical and Computer Engineering, Iowa State University, Ames, IA. [email protected]
Website: http://uigelz.ece.iastate.edu
33rd IEEE International Conference on Plasma ScienceTraverse City, MI June 4 – 8, 2006
*Supported by the NSF and 3M, Inc.ICOPS_2006
Iowa State University
Optical and Discharge Physics
AGENDA
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Plasma Surface Modification of Polymers
Description of the Model
Atmospheric Pressure He/O2/H2O Corona Discharges for Polypropylene Treatment
Gas flow
Pulsing frequency
Web speed
Concluding remarks
Polymers are used in variety of applications from textile apparel to packaging to biomedical materials.
The specific polymeric material is chosen not only for its bulk properties but also for surface characteristics such as wettability, adhesion and surface reactivity.
Iowa State UniversityOptical and Discharge Physics
APPLICATIONS OF POLYMERS
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Biomedical filtration Packaging material Textiles
The poor wettability and adhesion properties of hydrocarbon polymers is due to their low surface energy and limits use.
Ideally, the surface energy should exceed the liquid by 2-10 mN/m.
Plasma treatment is an effective dry process alternative to liquid chemical processes used to functionalize or activate the surface.
Iowa State UniversityOptical and Discharge PhysicsICOPS_2006_Ananth_4
Poor wettability-low surface energy
SURFACE PROPERTIES OF POLYMERS
FUNCTIONALIZATION OF POLYMER SURFACES
Functionalization occurs by the chemical interaction of plasma produced species - ions, radicals and photons with the surface.
Chemical groups are incorporated onto the surface which change surface properties.
Process usually only treats the top mono-layers not affecting the bulk.
Wettability on PE film with 3 zones of treatment: a)untreated b)slightly treated c) strongly treated.
Courtesy: http://www.polymer-surface.com
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(a)(b)
(c)
Iowa State University
Optical and Discharge Physics
PLASMA TREATMENT IMPROVES ADHESIVE BONDING
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Peel strength of Polyethylene (PE) downstream of an atmospheric pressure air non-equilibrium discharge.
M.J. Shenton et al, J. Phys D. 34, 2754 (2001)
Pee
l Str
engt
h (M
Pa)
Time (mins)
No Treatment
Adhesion strength of PE improves by a factor of 2-3 within a few seconds of treatment in an air plasma.
Adhesion shows some atmospheric degradation indicating long term reactivity.
Pulsed atmospheric filamentary discharges (coronas) are routinely used to web treat commodity polymers like polypropylene (PP) and polyethylene (PE).
Iowa State UniversityOptical and Discharge Physics
INDUSTRIAL SURFACE MODIFICATION OF POLYMERS
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TYPICAL CONDITIONS kVs at few kHz ~ few msWeb speed few m/s Gap : few mm
Filamentary Plasma 10s – 200 m
Advantages:
No vacuum equipment required.
Suitable for high throughput and continuous operation.
Economical.
Iowa State UniversityOptical and Discharge Physics
COMMERCIAL CORONA PLASMA EQUIPMENT
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Sigma, Inc.
Disadvantages:
Lack of specificity - mix of functional groups are produced.
Higher probability of surface contamination.
Most commonly treated polymer is polypropylene (PP).
Tantec, Inc.
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Optical and Discharge Physics
STRUCTURE OF POLYPROPYLENE
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Polypropylene (PP) is a saturated hydrocarbon polymer containing alternating methyl (-CH3) and H at the carbon centers on the backbone.
A Carbon atom can be attached to 3 H atoms (primary Carbon), 2 H atoms (secondary Carbon) or 1 H atom (tertiary Carbon).
The reactivity of the H depends on the C to which it is bonded, scaling as HT > HS > HP.
The surface site density of PP is about 1015/cm2 C-atoms.
PP undergoes surface oxidation in O2 containing discharges such as in air.
Coverage of O-containing groups is near 2.5% (2 x 1013 cm-2) for high energy density treatment and < 1% (<1013 cm-2) at lower energies.
Iowa State UniversityOptical and Discharge Physics
TREATMENT OF PP IN CORONA DISCHARGES
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Ref: O’Hare et al, Surf. Interface Anal. 33, 335–342 (2002)
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PROCESSING “HIGH-VALUE” PRODUCTS
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Biomedical materials are treated in (expensive) low pressure plasmas to selectively enhance cell adhesion or chemical reactivity to a reagent.
The drawback in using atmospheric pressure discharges is the lack of functional group specificity.
Improved control over incorporation of functional groups onto surfaces would enable use of commodity polymer processing techniques for high-value products with significant cost-savings.
Micropatterned cell growth on amino-functionalized polystyrene in NH3 and H2 plasmas
Ref: K. Schroeder et al, Plasmas and Polymers 7,103-125 (2002)
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GOALS OF THIS INVESTIGATION
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Results from 2-d modeling investigation of plasma and surface processes for polymer treatment will discuss degree and uniformity of surface functionalization.
Spatial dynamics of repetitively pulsed discharges.
Interplay between radical generation, transport and surface treatment processes
Gas flow and composition
Web speed
Pulsing frequency
Applied voltage
How do process variables ultimately affect the relative abundance of various surface functional groups?
Fully implicit solution of Poisson’s equation.
Continuity: Multi-fluid charged species equations using modified Scharfetter-Gummel fluxes.
Surface charge on dielectric surfaces.
2-d unstructured mesh.Iowa State University
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MODEL – ELECTROSTATICS, CHARGED PARTICLE TRANSPORT
iiis tNqt-t )()(
ii St
N
ICOPS_2006_Ananth_13
iEiii
S jqt
1
)exp(1
)exp(12/1 x
xnnD ii
i
D
vxq
q ii 21
Iowa State University
Optical and Discharge Physics
ELECTRON TRANSPORT AND REACTION KINETICS
Electron energy transport:
Reaction Kinetics include sources due to electron impact and heavy
particle reactions, photoionization and contributions from secondary emission.
eeeeeee TTkTTLTStkTn
2
5/
2
3
ICOPS_2006_Ananth_14
2
3
4
exp)()(
)(rr
rdrr
rNrN
rSjiji
Pi
j
jijSi jjS ,
Iowa State University
Optical and Discharge Physics
Fluid averaged values of mass density, mass momentum and thermal energy density obtained in using unsteady algorithms.
Continuity :
Momentum:
Energy :
Individual neutral species densities are updated.
)pumps,inlets()v(t
i
iii ENqvvNkTt
v
i i
iiiipp EjHRvPTcvTt
Tc
SV
T
iTiii SS
N
ttNNDvtNttN
FLUID MODULE : NEUTRAL PARTICLE TRANSPORT
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SURFACE KINETICS MODULE
To predict surface compositions, a surface kinetics module is incorporated into the plasma dynamics model.
Module predicts densities of surface resident groups using fluxes from the plasma and a user-provided mechanism.
Plasma Dynamics
Model
Fluxes
Surface densities of functional
groups
Sticking coefficients
Surface Kinetics Model
Surface reaction mechanism
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Electrode embedded in dielectric with tip exposed to the processing gas with a gap of 2 mm to the PP surface.
Atmospheric pressure
Applied voltage (10 ns pulses) at up to 10s kV, 0.1 – 10 kHz.
CORONA DISCHARGE GEOMETRY
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Not to scale
2 m
m
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Optical and Discharge Physics
GAS PHASE CHEMISTRY: He/O2/H2O
Treatment in O2 containing plasmas is known to effectively incorporate O atoms into the surface.
Process is initiated by electron impact dissociation of O2 and H2O into radicals such as O and OH.
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DYNAMICS OF THE FIRST PULSE: Te, SOURCES
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0–2 ns, no flow
Animation Slide-GIF
MIN MAX log scale
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Te 0-9 eV
Electron Source 5x1020-5x1023 cm-3s-1
Te peaks at the ionization front initiated near the electrode and propagates toward the PP surface.
Electron sources by electron impact ionization track the maximum in Te.
Iowa State University
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- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0–2 ns, no flow.
O 1011 – 1015 cm-3 OH 1011 – 1014 cm-3
PLASMA DYNAMICS OF THE FIRST PULSE
Electron density of 1013-1014 cm-3 is produced behind the front.
O and OH are produced predominantly by electron impact reactions of O2 and H2O respectively.
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Animation Slide-GIF
[e] 1011 – 1014 cm-3
MIN MAX log scale
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END OF FIRST PULSE AFTERGLOW: RADICALS
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 100 s, no flow
The density of O decreases to 1012 cm-3 in the interpulse period as it is consumed in 3-body reactions with O2 to form O3
(1014 cm-3).
The density of OH decreases to 1012 as it reacts with both O and O3.
ICOPS_2006_Ananth_21MIN MAX
log scale
[O3] 5x1012-5 x 1014 cm-3 [OH] 1011 – 1013 cm-3 [O] 1011 – 1013 cm-3
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RADICALS AND GROUPS AT CARBON CENTERS ON PP
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Polypropylene structure
Different radicals and functional groups are created at the carbon atoms when treated in O2
containing plasmas:
Alkyl Alkoxy Carbonyl Alcohol Peroxy Acid
R* R O* R = O R OH R O O* O = R OH
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SURFACE REACTION MECHANISM: INITIATION
Initiation by H abstraction:
Alkyl radicals (R*) formed by H abstraction by OH and O.
Propagation and saturation:
Peroxy (R-O-O*) formed by the addition of O2 to alkyl (R*) sites.
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= 1 - 10 s
= 10-100 s
Propagation:
Alkoxy (R-O*) formed by reaction of O3 and O with alkyl (R*) sites.
Surface – surface reactions:
Alkoxy (R-O*) radicals abstract H from surrounding sites to form alcohol (R-OH) groups.
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SURFACE REACTION MECHANISM: PROPAGATION
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= 10-100 s
= 10-50 ms
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SURFACE REACTION MECHANISM: CHAIN SCISSION
Carbonyl (R-C=O) groups are formed by chain scission.
Abstraction from carbonyl groups (R-C*=O) may lead to further chain degradation evolving CO2 into the gas phase.
ICOPS_2006_Ananth_25
= 50 - 100 ms
= 100 - 1000 ms
Termination
Addition of OH produces carboxylic acid groups.
H and OH also add to alkyl radicals (R*) in termination steps.
Iowa State UniversityOptical and Discharge Physics
SURFACE REACTION MECHANISM: TERMINATION
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= 100 - 1000 ms
• R* + O, O3 R - O* + O2
• R* + O2 R - OO*
Iowa State UniversityOptical and Discharge Physics
PP TREATMENT WITH A SINGLE PULSE
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 100 sICOPS_2006_Ananth_27
Alkyl (R*) radicals are formed within 10 s.
Alkoxy (R-O*) and peroxy (R-OO*) are
formed as alkyl (R*) sites react over 10s s .
R-OO*
R-O*
0.5 cm
R*
• RH + O, OH R* + OH, H2O
O and OH are generated in each pulse and consumed between pulses in reactions with O2 and O3 respectively.
O3 is relatively unreactive and so accumulates pulse-to-pulse.
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DYNAMICS WITH REPETITIVE PULSING (NO FLOW)
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 1 kHz, 0.005 s
ICOPS_2006_Ananth_28
OH
O3
O
[e]
10 cm
Animation Slide-GIF
1010 1014 cm-3, log scale
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 1 kHz, 0.05 s Iowa State UniversityOptical and Discharge Physics
PP TREATMENT WITH REPETITIVE PULSE (NO FLOW)
ICOPS_2006_Ananth_29
2 cm
• RH + O, OH R* + OH, H2O
Alkyls (R*) are regenerated every pulse by O and OH, and consumed.
Peroxy (R-O-O*) accumulate pulse-to-pulse
• R* + O2 R-O-O*
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PULSED DISCHARGES WITH GAS FLOW
- 5 kV, 1 atm, He/O2/H2O=89/10/1, few slpm ( | | = 10s – 100s cm/s)
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Axial gas flow varied from negligible to a few slpm ( = 10s ms)
How does gas flow aid in treatment downstream?
v
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EFFECT OF GAS FLOW ON RADICALS: [O]
no flow
10 slpm
30 slpm
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.005 s, 1 kHz, static surface
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O is highly reactive with O2 to form ozone (O3).
Although some O is convectively transported downstream, local reaction kinetics dominate. Nearly all O reacts prior to the next pulse.
1010 1014 cm-3, log scale
Animation Slide-GIF
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EFFECT OF GAS FLOW ON RADICALS: [O3]
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.005 s, 1 kHz, static surface
ICOPS_2006_Ananth_32
no flow
10 slpm
30 slpm
With gas flow, the accumulating O3 is convected downstream.
Animation Slide-GIF
1010 1014 cm-3, log scale
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EFFECT OF GAS FLOW ON PP TREATMENT
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Alkoxy (R-O*) and alcohol (R-OH) decrease under the electrode.
Peroxy (R-O-O*) increases downstream as alkyl sites are saturated.
10 cm
• R-OH • R-OO*
• R* + O2 R-O-O*
• R* + O3 R - O* R-OH
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.05 s, 1 kHz, static surface
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WEB TREATMENT OF POLYMER SURFACES
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TYPICAL CONDITIONS ~ few ms Gap : few mm
Polymer surfaces are continuously treated at web speeds of a few m/s.
Model addresses web treatment by translate the surface properties on the grid at a few m/s.Moving surface
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CONTINUOUS TREATMENT
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Surface has active sites which react downstream of the plasma zone.
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0-0.025s, 1 kHz, web speed = 4 m/s, no flow
• R-O*
Moving surface
10 cm
• R-OH
Moving surface
Moving surface
• R* + O2 R-O-O*• R* + O3 R - O* R-OH
• R*
Moving surface
• R*
Moving surface
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CONTINUOUS TREATMENT: GAS FLOW
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0.05 s, 1 kHz, film spd = 4 m/s
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Gas flow reduces alkoxy (R-O*) and alcohol (R-OH) coverage and increases peroxy (R-O-O*) by altering relative fluxes of O and O3.
10 cm
Moving surface
R-OHR-OO*
No flow
10 slpm
No flow
Moving surface Moving surface
• R* + O2 R-O-O*• R* + O3 R - O* R-OH
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CONTINUOUS TREATMENT: SURFACE RESIDENCE TIME
- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.05 s, 1 kHz
ICOPS_2006_Ananth_37
Lower web speeds improves uniformity by averaging out pulse-to-pulse modulation.
R-OH
Moving surface
R-OO*
10 cm
Moving surface
Moving surface
• R* + O2 R-O-O*• R* + O3 R - O* R-OH
Use of reactive gases (such as NH3) in room-air environments require sophisticated gas injection and confinement.
Iowa State UniversityOptical and Discharge Physics
USE OF REACTIVE GAS MIXTURES
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F. Forster et al, Surf. Coatings Technol., 98, 1121 (1998). J. F. Behnke et al, Vacuum, 71, 417 (2003).
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SHOE ELECTRODE CONFIGURATION
Alternating positive and negative 15 kV pulses.
Gap = 2 mm.
He/O2 flow injected into an air environment at a few slpm.
Continuous processing with moving web.
Seed electrons randomly with Gaussian distribution.
ICOPS_2006_Ananth_39
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REPETITIVELY PULSED DISCHARGE DYNAMICS: [e]
Peak electron densities (1014 cm-3) are generated adjacent to the momentary cathode.
Evidence of “sparking” at edge of electrode.
ICOPS_2006_Ananth_40
-15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm
Animation Slide-GIF
[e] 1010 – 1014 cm-3
1010 1014 cm-3, log scale
Air
He/O2
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REPETITIVELY PULSED DISCHARGE DYNAMICS – [O]
Electron impact dissociation of O2 produces “delta function” sources of O.
In the interpulse period, O is consumed in formation of O3 while being convected downstream.
ICOPS_2006_Ananth_40
-15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm
Animation Slide-GIF
O 1011 – 1015 cm-3
1011 1015 cm-3, log scale
Air
He/O2
O3 is generated pulse to pulse, accumulate in discharge and is convected downstream.
Iowa State UniversityOptical and Discharge Physics
REPETITIVELY PULSED DISCHARGE DYNAMICS – [O3]
ICOPS_2006_Ananth_40
-15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm
Animation Slide-GIF
O3
1012 – 1016 cm-3
Air
He/O2
1012 1015 cm-3, log scale
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CONTINUOUS PROCESSING OF PP
ICOPS_2006_Ananth_41
The PP is functionalized by successive pulses as it moves through the discharge.
Peroxy (R-O-O*) coverage increase towards the exit due to cumulative exposure.
- 15 kV, 1 atm, He/O2/H2O=90/10, 0 – 0.05 s, 1 kHz, 10 slpm
Moving surface
• R* + O2 R-O-O*
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CONCLUDING REMARKS
Optimization of polymer treatment using commercial corona equipment could lead to creating high value materials.
Control of process variables (eg., gas flow, mixture, web-speed) may enable production of unique surface compositions.
In PP treatment, relative fluxes of reactive species is altered by gas flow changing the abundance of alkoxy (R-O*) and peroxy (R-O-O*).
Ultimately, customization of surfaces must account for
Reactive radicals (e.g., O and OH) are regenerated each pulse; longer lived (e.g., O3) accumulate over many pulses.
Gas flow transports long lived radicals over more surface area. Moving speed “mixes” of two regimes.
Interplay between local rapid reactions and non-local slower reactions may enable customization.
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