RADICAL GENERATION AND POLYMER SURFACE FUNCTIONALIZATION IN FLOWING ATMOSPHERIC PRESSURE PULSED DISCHARGES* Ananth N. Bhoj a) and Mark J. Kushner b) a)

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

ICOPS_2006_Ananth_2

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

ICOPS_2006_Ananth_3

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

ICOPS_2006_Ananth_5


(a)(b)

(c)



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).


INDUSTRIAL SURFACE MODIFICATION OF POLYMERS

ICOPS_2006_Ananth_7

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.


COMMERCIAL CORONA PLASMA EQUIPMENT

ICOPS_2006_Ananth_8

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.



STRUCTURE OF POLYPROPYLENE

ICOPS_2006_Ananth_9

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.


TREATMENT OF PP IN CORONA DISCHARGES

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Ref: O’Hare et al, Surf. Interface Anal. 33, 335–342 (2002)



PROCESSING “HIGH-VALUE” PRODUCTS


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)



GOALS OF THIS INVESTIGATION


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


MODEL – ELECTROSTATICS, CHARGED PARTICLE TRANSPORT

iiis tNqt-t )()(

ii St

N


iEiii

S jqt

1

)exp(1

)exp(12/1 x

xnnD ii

i

D

vxq

q ii 21



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


2

3

4

exp)()(

)(rr

rdrr

rNrN

rSjiji

Pi

j

jijSi jjS ,



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



Optical and Discharge PhysicsICOPS_2006_Ananth_16

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



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


Not to scale

2 m

m



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.




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


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.



- 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.


Animation Slide-GIF

[e] 1011 – 1014 cm-3

MIN MAX log scale



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



RADICALS AND GROUPS AT CARBON CENTERS ON PP


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



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.


= 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.


SURFACE REACTION MECHANISM: PROPAGATION


= 10-100 s

= 10-50 ms



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.


= 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.


SURFACE REACTION MECHANISM: TERMINATION


= 100 - 1000 ms

• R* + O, O3 R - O* + O2

• R* + O2 R - OO*


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.


DYNAMICS WITH REPETITIVE PULSING (NO FLOW)

- 5 kV, 1 atm, He/O2/H2O=89/10/1, 1 kHz, 0.005 s


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)


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*



PULSED DISCHARGES WITH GAS FLOW

- 5 kV, 1 atm, He/O2/H2O=89/10/1, few slpm ( | | = 10s – 100s cm/s)


Axial gas flow varied from negligible to a few slpm ( = 10s ms)

How does gas flow aid in treatment downstream?

v



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


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.


Animation Slide-GIF



EFFECT OF GAS FLOW ON RADICALS: [O3]



no flow

10 slpm

30 slpm

With gas flow, the accumulating O3 is convected downstream.

Animation Slide-GIF




EFFECT OF GAS FLOW ON PP TREATMENT


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




WEB TREATMENT OF POLYMER SURFACES


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



CONTINUOUS TREATMENT


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



CONTINUOUS TREATMENT: GAS FLOW

- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0.05 s, 1 kHz, film spd = 4 m/s


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



CONTINUOUS TREATMENT: SURFACE RESIDENCE TIME

- 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.05 s, 1 kHz


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.


USE OF REACTIVE GAS MIXTURES


F. Forster et al, Surf. Coatings Technol., 98, 1121 (1998). J. F. Behnke et al, Vacuum, 71, 417 (2003).



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.




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.


-15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm

Animation Slide-GIF

[e] 1010 – 1014 cm-3


Air

He/O2



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.



Animation Slide-GIF

O 1011 – 1015 cm-3


Air

He/O2

O3 is generated pulse to pulse, accumulate in discharge and is convected downstream.


REPETITIVELY PULSED DISCHARGE DYNAMICS – [O3]



Animation Slide-GIF

O3

1012 – 1016 cm-3

Air

He/O2




CONTINUOUS PROCESSING OF PP


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*



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.


Documents

RADICAL GENERATION AND POLYMER SURFACE FUNCTIONALIZATION IN FLOWING ATMOSPHERIC PRESSURE PULSED DISCHARGES* Ananth N. Bhoj a) and Mark J. Kushner b) a)