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ICOPS_2006

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. bhoj@uiuc.edu

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ICOPS_2006

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  1. 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. bhoj@uiuc.edu b)Department of Electrical and Computer Engineering, Iowa State University, Ames, IA. mjk@iastate.edu Website: http://uigelz.ece.iastate.edu 33rd IEEE International Conference on Plasma Science Traverse City, MI June 4 – 8, 2006 *Supported by the NSF and 3M, Inc. ICOPS_2006

  2. AGENDA  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 Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_2

  3. APPLICATIONS OF POLYMERS  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.  Packaging material  Textiles  Biomedical filtration Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_3

  4. SURFACE PROPERTIES OF POLYMERS  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. Poor wettability-low surface energy Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_4

  5. Electrical Discharge • An electrical discharge is the passage of electrical current through a material which normally does not conduct electricity. Consider, for example, a simple experiment which we have all experienced. If we hold two wires a few millimeters apart, and connect each to one pole of a battery, no perceptible electrical current flows through the air, because the air is insulating. However if these wire were connected to a high voltage source of several thousand volts, sparks will fly. The normally insulating air was transformed into a conductor, a process called electrical breakdown, and the sparks which we would see are a form of an electrical discharge. • Normally air consists of neutral molecules of nitrogen, oxygen, and other gases, in which electrons are tightly bound to atomic nuclei. During the breakdown process, some of the negatively charged electrons are separated from their host atoms, leaving them with a positive charge. The negatively charged electrons, and the positively charged atoms (known as positive ions) are then free to move separately under the influence of the applied voltage. Their movement constitutes an electrical current. • The collection of ions and electrons is known as a plasma, and one of its more important properties is that a plasma can conduct electrical current.

  6. Plasma • There are several types of electrical discharges: • The Corona is a 'partial' discharge occurring when a highly inhomogeneous electric field is imposed. Typically, there is a very high electric field adjacent to a sharp electrode, and a net production of new electron-ion pairs occurs in this vicinity. The Corona typically has a very low current, and very high voltage. • The Glow Discharge typically has a voltage of several hundred volts, and currents up to 1 A. A small electron current is emitted from the cathode by collisions of ions, excited atoms, and photons, and then multiplied by successive electron impact ionization collisions in the cathode fall region. • The Arc is a high current, low voltage discharge, where electron emission from the cathode is from thermionic and/or field emission. Electrical discharges can also by excited by RF, microwave, or laser radiation, with or without electrodes.

  7. Theory of Corona Treating • OXIDATION: Basically, this theory states that the energy of the corona breaks the molecular bonds on the surface of the non-polar substrate. The broken bonds then recombine with the free radicals in the corona environment to form additional polar groups on the film surface. These polar groups have a strong chemical affinity to the polar inks and adhesives, which results in improved adhesion. Similarly, the polar surface results in an increased surface energy which correlates with improved wettability. • MICROPITTING: The micropitting theory states that the surface of the material being treated is pittted, increasing the surface area and giving it a better surface for the coating or lamination to grab onto. • THE ELECTRET EFFECT: The electret theory describes a process within the corona where the polar chain of the polymer being treated is carbonized giving it a chemically reactive surface for the coating or lamination to bond to.

  8. (b) (a) (c) 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 Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_5

  9. Peel Strength (MPa) No Treatment Time (mins) PLASMA TREATMENT IMPROVES ADHESIVE BONDING • 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. • 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) Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_6

  10. INDUSTRIAL SURFACE MODIFICATION OF POLYMERS  Pulsed atmospheric filamentary discharges (coronas) are routinely used to web treat commodity polymers like polypropylene (PP) and polyethylene (PE). TYPICAL CONDITIONS  kVs at few kHz  t ~ few ms Web speed few m/s  Gap : few mm  Filamentary Plasma 10s – 200 mm Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_7

  11. Corona- Basic Concept • A corona treatment system in its simplest form can be thought of as a capacitor. Voltage is applied to the top plate which, in the case of corona treatment, would be the electrode.  The dielectric portion of the capacitor would be made up of some type of roll covering, air and the substrate (film or sheet).  The final component, or bottom plate, would take the form of an electrically grounded roll.  In the corona treatment system the voltage build-up ionises the air in the air gap creating a corona, which modifies the surface and increases the surface energy of the substrate passing over the electrically grounded roll.  The level of treatment is controlled by the energy of the discharge and the air gap.  For health and safety reasons, the ozone generated in the corona must be removed from the working environment.

  12. Covered Roll • Covered roll stations have the dielectric covering on the ground roll and the high voltage electrode is bare metal.

  13. Bare Roll • Bare roll stations have the dielectric covering on the high voltage electrode and the ground electrode is bare metal.

  14. COMMERCIAL CORONA PLASMA EQUIPMENT • Sigma, Inc. • Tantec, Inc. Advantages:  No vacuum equipment required.  Suitable for high throughput and continuous operation.  Economical. Disadvantages:  Lack of specificity - mix of functional groups are produced.  Higher probability of surface contamination.  Most commonly treated polymer is polypropylene (PP). Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_8

  15. STRUCTURE OF POLYPROPYLENE  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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_9

  16. TREATMENT OF PP IN CORONA DISCHARGES  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. • Ref: O’Hare et al, Surf. Interface Anal. 33, 335–342 (2002) Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_10

  17. 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. • Micropatterned cell growth on amino-functionalized polystyrene in NH3 and H2 plasmas  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. • Ref: K. Schroeder et al, Plasmas and Polymers 7,103-125 (2002) Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_11

  18. 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? Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_12

  19. MODEL – ELECTROSTATICS, CHARGED PARTICLE TRANSPORT  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 ICOPS_2006_Ananth_13

  20. 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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_14

  21. FLUID MODULE : NEUTRAL PARTICLE TRANSPORT • 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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_15

  22. Fluxes Surface densities of functional groups Surface Kinetics Model Plasma Dynamics Model Sticking coefficients Surface reaction mechanism 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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_16

  23. Components of Corona Treating Systems • Power Source • The power source generally consists of a high frequency generator and a high voltage output transformer. In very general terms, the purpose of the power source is to raise the incoming electricity (typically 50/60 Hz, 230/460 V) to a higher frequency (10-35 kHz) and higher voltage (10 kV). The power source is commonly referred to as a power supply or a generator. Typically, power supplies are rated in kilowatts (kW) and can range from 500 W to 30 kW, depending on the application. • Treater Station • All treater stations have a high voltage electrode and a ground electrode. A solid dielectric (insulating) material is needed to cover one of the two electrodes in order to generate a corona atmosphere, as opposed to a "lightening bolt" charge (the dielectric covering (silicone, ceramic, epoxy, etc.) prevents the voltage from arcing to the ground roll. Instead the air is broken down and a corona (oxidised air) is generated). Heat, ozone and NOX are formed.

  24. 2 mm Not to scale CORONA DISCHARGE GEOMETRY  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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_17

  25. 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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_18

  26. MINMAX log scale DYNAMICS OF THE FIRST PULSE: Te, SOURCES • 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.  Te0-9 eV Electron Source 5x1020-5x1023 cm-3s-1 Animation Slide-GIF - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0–2 ns, no flow Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_19

  27. MINMAX log scale PLASMA DYNAMICS OF THE FIRST PULSE  [e] 1011 – 1014 cm-3  OH 1011 – 1014 cm-3  O 1011 – 1015 cm-3 • 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. - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0–2 ns, no flow. Animation Slide-GIF Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_20

  28. MINMAX log scale END OF FIRST PULSE AFTERGLOW: RADICALS  [O] 1011 – 1013 cm-3  [OH] 1011 – 1013 cm-3  [O3] 5x1012-5 x 1014 cm-3 • 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. - 5 kV, 1 atm, He/O2/H2O=89/10/1, 100 ms, no flow Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_21

  29. RADICALS AND GROUPS AT CARBON CENTERS ON PP  Polypropylene structure  Different radicals and functional groups are created at the carbon atoms when treated in O2containing 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 ICOPS_2006_Ananth_22

  30. 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. t = 1 - 10 ms t = 10-100 ms Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_23

  31. t = 10-100 ms t = 10-50 ms SURFACE REACTION MECHANISM: PROPAGATION  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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_24

  32. t = 50 - 100 ms t = 100 - 1000 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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_25

  33. t = 100 - 1000 ms SURFACE REACTION MECHANISM: TERMINATION  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 ICOPS_2006_Ananth_26

  34. R* 0.5 cm PP TREATMENT WITH A SINGLE PULSE • R* + O, O3 R - O* + O2 • R* + O2 R - OO* • RH + O, OH  R* + OH, H2O R-OO* R-O*  Alkyl (R*) radicals are formed within 10 ms. Alkoxy(R-O*) and peroxy (R-OO*) are formed as alkyl (R*) sites react over 10s ms . - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 100 ms Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_27

  35. 1010 1014 cm-3, log scale DYNAMICS WITH REPETITIVE PULSING (NO FLOW) 10 cm • [e]  O  OH • 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.  O3 Animation Slide-GIF - 5 kV, 1 atm, He/O2/H2O=89/10/1, 1 kHz, 0.005 s Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_28

  36. 2 cm PP TREATMENT WITH REPETITIVE PULSE (NO FLOW) • RH + O, OH  R* + OH, H2O • R* + O2 R-O-O* • Alkyls (R*) are regenerated every pulse by O and OH, and consumed. • Peroxy (R-O-O*) accumulate pulse-to-pulse - 5 kV, 1 atm, He/O2/H2O=89/10/1, 1 kHz, 0.05 s Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_29

  37. PULSED DISCHARGES WITH GAS FLOW  Axial gas flow varied from negligible to a few slpm (t = 10s ms)  How does gas flow aid in treatment downstream? - 5 kV, 1 atm, He/O2/H2O=89/10/1, few slpm ( | | = 10s – 100s cm/s) Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_30

  38. 1010 1014 cm-3, log scale EFFECT OF GAS FLOW ON RADICALS: [O]  no flow  10 slpm  30 slpm • 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 - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.005 s, 1 kHz, static surface Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_31

  39. 1010 1014 cm-3, log scale 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 - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.005 s, 1 kHz, static surface Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_32

  40. EFFECT OF GAS FLOW ON PP TREATMENT • R* + O2 R-O-O* • R* + O3 R - O*  R-OH 10 cm • R-OH • R-OO* • Alkoxy (R-O*) and alcohol (R-OH) decrease under the electrode. • Peroxy (R-O-O*) increases downstream as alkyl sites are saturated. - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.05 s, 1 kHz, static surface Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_33

  41. Moving surface WEB TREATMENT OF POLYMER SURFACES • 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. TYPICAL CONDITIONS  t ~ few ms  Gap : few mm Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_34

  42. R-O* Moving surface 10 cm • R-OH Moving surface • R* • R* Moving surface 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 Moving surface • R* + O2 R-O-O* • R* + O3 R - O*  R-OH Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_35

  43. CONTINUOUS TREATMENT: GAS FLOW Moving surface • R* + O2 R-O-O* • R* + O3 R - O*  R-OH 10 cm R-OO* R-OH No flow No flow 10 slpm Moving surface Moving surface • 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.  - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0.05 s, 1 kHz, film spd = 4 m/s Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_36

  44. Moving surface 10 cm R-OO* R-OH Moving surface Moving surface CONTINUOUS TREATMENT: SURFACE RESIDENCE TIME • R* + O2 R-O-O* • R* + O3 R - O*  R-OH • Lower web speeds improves uniformity by averaging out pulse-to-pulse modulation. - 5 kV, 1 atm, He/O2/H2O=89/10/1, 0 – 0.05 s, 1 kHz Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_37

  45. USE OF REACTIVE GAS MIXTURES Use of reactive gases (such as NH3) in room-air environments require sophisticated gas injection and confinement. • 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 ICOPS_2006_Ananth_38

  46. 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. Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_39

  47. 1010 1014 cm-3, log scale REPETITIVELY PULSED DISCHARGE DYNAMICS: [e] He/O2 • Peak electron densities (1014 cm-3) are generated adjacent to the momentary cathode. • Evidence of “sparking” at edge of electrode. Air  [e] 1010 – 1014 cm-3 Animation Slide-GIF -15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_40

  48. 1011 1015 cm-3, log scale REPETITIVELY PULSED DISCHARGE DYNAMICS – [O] He/O2 • 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. Air  O 1011 – 1015 cm-3 Animation Slide-GIF -15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_40

  49. 1012 1015 cm-3, log scale REPETITIVELY PULSED DISCHARGE DYNAMICS – [O3] He/O2 Air  O3 is generated pulse to pulse, accumulate in discharge and is convected downstream.  O3 1012 – 1016 cm-3 Animation Slide-GIF -15 kV, 1 atm, He/O2=90/10, 0 – 0.005 s, 1 kHz, 10 slpm Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_40

  50. Moving surface CONTINUOUS PROCESSING OF PP • R* + O2 R-O-O* • 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 Iowa State University Optical and Discharge Physics ICOPS_2006_Ananth_41

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