RADICAL GENERATION AND POLYMER SURFACE FUNCTIONALIZATION IN FLOWING ATMOSPHERIC PRESSURE PULSED DISCHARGES* Ananth N. Bhoj a) and Mark J. Kushner b) a) Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, IL. b) Department of Electrical and Computer Engineering, Iowa State University, Ames, IA. Website: 33 rd IEEE International Conference on Plasma Science Traverse 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/O 2 /H 2 O 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 University Optical 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 University Optical and Discharge Physics ICOPS_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: ICOPS_2006_Ananth_5 Iowa State University Optical and Discharge Physics (a) (b) (c)

Iowa State University Optical and Discharge Physics PLASMA TREATMENT IMPROVES ADHESIVE BONDING ICOPS_2006_Ananth_6  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) Peel Strength (MPa) 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 University Optical and Discharge Physics INDUSTRIAL SURFACE MODIFICATION OF POLYMERS ICOPS_2006_Ananth_7 TYPICAL CONDITIONS  kVs at few kHz   ~ few ms   Web 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 University Optical and Discharge Physics 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.

Iowa State University Optical and Discharge Physics STRUCTURE OF POLYPROPYLENE ICOPS_2006_Ananth_9  Polypropylene (PP) is a saturated hydrocarbon polymer containing alternating methyl (-CH 3 ) 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 H T > H S > H P.  The surface site density of PP is about /cm 2 C-atoms.

 PP undergoes surface oxidation in O 2 containing discharges such as in air.  Coverage of O-containing groups is near 2.5% (2 x cm -2 ) for high energy density treatment and < 1% (<10 13 cm -2 ) at lower energies. Iowa State University Optical and Discharge Physics TREATMENT OF PP IN CORONA DISCHARGES ICOPS_2006_Ananth_10  Ref: O’Hare et al, Surf. Interface Anal. 33, 335–342 (2002)

Iowa State University Optical and Discharge Physics PROCESSING “HIGH-VALUE” PRODUCTS ICOPS_2006_Ananth_11  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 NH 3 and H 2 plasmas  Ref: K. Schroeder et al, Plasmas and Polymers 7, (2002)

Iowa State University Optical and Discharge Physics GOALS OF THIS INVESTIGATION ICOPS_2006_Ananth_12  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 Optical and Discharge Physics MODEL – ELECTROSTATICS, CHARGED PARTICLE TRANSPORT ICOPS_2006_Ananth_13

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. ICOPS_2006_Ananth_14

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. FLUID MODULE : NEUTRAL PARTICLE TRANSPORT ICOPS_2006_Ananth_15

Iowa State University Optical and Discharge Physics ICOPS_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

Iowa State University Optical and Discharge Physics  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 ICOPS_2006_Ananth_17 Not to scale 2 mm

Iowa State University Optical and Discharge Physics GAS PHASE CHEMISTRY: He/O 2 /H 2 O  Treatment in O 2 containing plasmas is known to effectively incorporate O atoms into the surface.  Process is initiated by electron impact dissociation of O 2 and H 2 O into radicals such as O and OH. ICOPS_2006_Ananth_18

Iowa State University Optical and Discharge Physics DYNAMICS OF THE FIRST PULSE: T e, SOURCES  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0–2 ns, no flow Animation Slide-GIF MIN MAX log scale ICOPS_2006_Ananth_19  T e 0-9 eV  Electron Source 5x x10 23 cm -3 s -1  T e 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 T e.

Iowa State University Optical and Discharge Physics  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0–2 ns, no flow.  O – cm -3  OH – cm -3 PLASMA DYNAMICS OF THE FIRST PULSE  Electron density of cm -3 is produced behind the front.  O and OH are produced predominantly by electron impact reactions of O 2 and H 2 O respectively. ICOPS_2006_Ananth_20 Animation Slide-GIF  [e] – cm -3 MIN MAX log scale

Iowa State University Optical and Discharge Physics END OF FIRST PULSE AFTERGLOW: RADICALS  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 100  s, no flow  The density of O decreases to cm -3 in the interpulse period as it is consumed in 3-body reactions with O 2 to form O 3 (10 14 cm -3 ).  The density of OH decreases to as it reacts with both O and O 3. ICOPS_2006_Ananth_21 MIN MAX log scale  [O 3 ] 5x x cm -3  [OH] – cm -3  [O] – cm -3

Iowa State University Optical and Discharge Physics RADICALS AND GROUPS AT CARBON CENTERS ON PP ICOPS_2006_Ananth_22  Polypropylene structure  Different radicals and functional groups are created at the carbon atoms when treated in O 2 containing plasmas: Alkyl Alkoxy Carbonyl Alcohol Peroxy Acid R* R  O* R = O R  OH R  O  O* O = R  OH

Iowa State University Optical and Discharge Physics 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 O 2 to alkyl (R*) sites. ICOPS_2006_Ananth_23  =  s  =  s

 Propagation: Alkoxy (R-O*) formed by reaction of O 3 and O with alkyl (R*) sites.  Surface – surface reactions: Alkoxy (R-O*) radicals abstract H from surrounding sites to form alcohol (R-OH) groups. Iowa State University Optical and Discharge Physics SURFACE REACTION MECHANISM: PROPAGATION ICOPS_2006_Ananth_24  =  s  = ms

Iowa State University Optical and Discharge Physics 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 CO 2 into the gas phase. ICOPS_2006_Ananth_25  = ms  = ms

 Termination Addition of OH produces carboxylic acid groups. H and OH also add to alkyl radicals (R*) in termination steps. Iowa State University Optical and Discharge Physics SURFACE REACTION MECHANISM: TERMINATION ICOPS_2006_Ananth_26  = ms

R* + O, O 3  R - O* + O 2 R* + O 2  R - OO * Iowa State University Optical and Discharge Physics PP TREATMENT WITH A SINGLE PULSE  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0 – 100  s ICOPS_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, H 2 O

 O and OH are generated in each pulse and consumed between pulses in reactions with O 2 and O 3 respectively.  O 3 is relatively unreactive and so accumulates pulse-to-pulse. Iowa State University Optical and Discharge Physics DYNAMICS WITH REPETITIVE PULSING (NO FLOW)  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 1 kHz, s ICOPS_2006_Ananth_28  OH  O 3  O  [e] 10 cm Animation Slide-GIF cm -3, log scale

 - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 1 kHz, 0.05 s Iowa State University Optical and Discharge Physics PP TREATMENT WITH REPETITIVE PULSE (NO FLOW) ICOPS_2006_Ananth_29 2 cm RH + O, OH  R* + OH, H 2 O  Alkyls (R*) are regenerated every pulse by O and OH, and consumed.  Peroxy (R-O-O*) accumulate pulse-to- pulse R* + O 2  R-O-O*

Iowa State University Optical and Discharge Physics PULSED DISCHARGES WITH GAS FLOW  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, few slpm ( | | = 10s – 100s cm/s) ICOPS_2006_Ananth_30  Axial gas flow varied from negligible to a few slpm (  = 10s ms)  How does gas flow aid in treatment downstream?

Iowa State University Optical and Discharge Physics EFFECT OF GAS FLOW ON RADICALS: [O]  no flow  10 slpm  30 slpm  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0 – s, 1 kHz, static surface ICOPS_2006_Ananth_31  O is highly reactive with O 2 to form ozone (O 3 ).  Although some O is convectively transported downstream, local reaction kinetics dominate. Nearly all O reacts prior to the next pulse cm -3, log scale Animation Slide-GIF

Iowa State University Optical and Discharge Physics EFFECT OF GAS FLOW ON RADICALS: [O 3 ]  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0 – s, 1 kHz, static surface ICOPS_2006_Ananth_32  no flow  10 slpm  30 slpm  With gas flow, the accumulating O 3 is convected downstream. Animation Slide-GIF cm -3, log scale

Iowa State University Optical and Discharge Physics EFFECT OF GAS FLOW ON PP TREATMENT ICOPS_2006_Ananth_33  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* + O 2  R-O-O* R* + O 3  R - O*  R-OH  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0 – 0.05 s, 1 kHz, static surface

Iowa State University Optical and Discharge Physics WEB TREATMENT OF POLYMER SURFACES ICOPS_2006_Ananth_34 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

Iowa State University Optical and Discharge Physics CONTINUOUS TREATMENT ICOPS_2006_Ananth_35  Surface has active sites which react downstream of the plasma zone.  - 5 kV, 1 atm, He/O2/H2O=89/10/1, s, 1 kHz, web speed = 4 m/s, no flow R-O* Moving surface 10 cm R-OH Moving surface R* + O 2  R-O-O* R* + O 3  R - O*  R-OH R* Moving surface R* Moving surface

Iowa State University Optical and Discharge Physics CONTINUOUS TREATMENT: GAS FLOW  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0.05 s, 1 kHz, film spd = 4 m/s ICOPS_2006_Ananth_36  Gas flow reduces alkoxy (R-O*) and alcohol (R-OH) coverage and increases peroxy (R-O-O*) by altering relative fluxes of O and O cm Moving surface R-OH R-OO* No flow 10 slpm No flow Moving surface R* + O 2  R-O-O* R* + O 3  R - O*  R-OH

Iowa State University Optical and Discharge Physics CONTINUOUS TREATMENT: SURFACE RESIDENCE TIME  - 5 kV, 1 atm, He/O 2 /H 2 O=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 R* + O 2  R-O-O* R* + O 3  R - O*  R-OH

 Use of reactive gases (such as NH 3 ) in room-air environments require sophisticated gas injection and confinement. Iowa State University Optical and Discharge Physics USE OF REACTIVE GAS MIXTURES ICOPS_2006_Ananth_38  F. Forster et al, Surf. Coatings Technol., 98, 1121 (1998).  J. F. Behnke et al, Vacuum, 71, 417 (2003).

Iowa State University Optical and Discharge Physics SHOE ELECTRODE CONFIGURATION  Alternating positive and negative 15 kV pulses.  Gap = 2 mm.  He/O 2 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

Iowa State University Optical and Discharge Physics REPETITIVELY PULSED DISCHARGE DYNAMICS: [e]  Peak electron densities (10 14 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/O 2 =90/10, 0 – s, 1 kHz, 10 slpm Animation Slide-GIF  [e] – cm cm -3, log scale Air He/O 2

Iowa State University Optical and Discharge Physics REPETITIVELY PULSED DISCHARGE DYNAMICS – [O]  Electron impact dissociation of O 2 produces “delta function” sources of O.  In the interpulse period, O is consumed in formation of O 3 while being convected downstream. ICOPS_2006_Ananth_40  -15 kV, 1 atm, He/O 2 =90/10, 0 – s, 1 kHz, 10 slpm Animation Slide-GIF  O – cm cm -3, log scale Air He/O 2

 O 3 is generated pulse to pulse, accumulate in discharge and is convected downstream. Iowa State University Optical and Discharge Physics REPETITIVELY PULSED DISCHARGE DYNAMICS – [O 3 ] ICOPS_2006_Ananth_40  -15 kV, 1 atm, He/O 2 =90/10, 0 – s, 1 kHz, 10 slpm Animation Slide-GIF  O – cm -3 Air He/O cm -3, log scale

Iowa State University Optical and Discharge Physics 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/O 2 /H 2 O=90/10, 0 – 0.05 s, 1 kHz, 10 slpm Moving surface R* + O 2  R-O-O*

Iowa State University Optical and Discharge Physics 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., O 3 ) 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. ICOPS_2006_Ananth_42