Ghent University Academic Bibliography

Advanced

Dielectric barrier discharge plasma jet for the surface treatment of polymeric substrates

Abdollah Sarani UGent (2012)
abstract
Plasma jets are sources of non-thermal or “cold” plasma that are increasingly used in several applications in materials technology and, more recently, in medicine. A key advantage of plasma jet systems is the fact that the available plasma volume is not limited to the gap between the electrodes. By means of a gas flow through the system, a “flowing afterglow” is created that is brought in contact with the surface to be treated. The dimensions of the flowing afterglow region can be set by the choice of gas, flow rate and geometry, which enables treating complex three-dimensional surfaces such as porous polymers and dental cavities. As the gas temperature in the afterglow region is relatively low, plasma jets are typically used in the surface treatment of heat-sensitive materials that can be exposed without the risk of thermal damage to the radicals (OH, O, ), metastables and UV-photons that are produced in the plasma. Although the under-standing of the detailed mechanisms is still incomplete, it can be stated in general terms that the mentioned “active species” are responsible for initiating the surface processes that lead to the envisaged plasma effect. It follows that when designing a plasma jet, a high flux of active species is targeted. One way of achieving that is by a proper choice of the gas mixture. In the literature the influence of adding small amounts of oxygen (O2 ) to the gas mixture is well documented. In this work two alternative additives, water vapor and CO2 are investigated. The first part of the thesis is a detailed characterization of a dielectric barrier discharge (DBD) plasma jet in argon, helium and mixtures of these gases with water vapor. The electrical properties of the plasma, i.e. the voltage-current characteristics and the power dissipated in the discharge, are determined from the voltage and current waveforms and from charge-voltage Lissajous figures respectively. The results indicate that when water vapor is added at constant ap-plied voltage, the power coupled to the discharge decreases. In ad-dition, in water-containing discharges the current waveform features an asymmetry between the negative and positive half cycle of the applied voltage. The length of the flowing afterglow of the plasma jet is pho-tographed under different experimental conditions (gas flow, mixture composition, applied voltage). For a helium discharge the length of the afterglow region first increases from 21 mm to 54 mm when the gas flow rate is increased from 0.695 slm to 3.4 slm. Upon a fur-ther increase of the gas flow rate to 5 slm, the afterglow shrinks to 43 mm, as a result of the transition of the flow regime from laminar to turbulent. Adding water vapor shortens the afterglow. The same trends are observed for argon. It is evidenced that the flowing after-glow of a plasma jet in helium consists of plasma bullets and that this type of plasma is sustained by photo-ionization processes. There is no difference in the plasma dynamics between the positive and neg-ative half cycle of the applied voltage. The propagation velocity of a plasma bullet is not influenced by the addition of water vapor in the 0.05%-0.76% concentration range. Time-resolved optical emission spectroscopy (OES) is applied to both the active plasma zone and the afterglow region. Atomic lines and molecular bands are identified, yielding a qualitative picture of the plasma composition. Important emission bands, including OH, are mapped with a spatial resolution of 1 mm. It is observed that when a small amount (0.5%) of water vapor is added to a pure argon discharge, the intensity of the OH-emission is increased. At higher water vapor concentrations the intensity drops. This can be explained by the quenching of OH-radicals by collisions with water molecules and by a decrease of the electron temperature as a result of inelastic collisions between electrons and molecular additives (i.c. water). The latter effect also explains the intensity drop of the atomic lines when water vapor is added to the mixture. A good estimate of the electron temperature can be obtained by measuring the excitation temperature of argon. The latter amounts to 0.95 eV in pure argon and is confirmed to decrease through the addition of water. From the current density a value for the electron density of 1.5E13 cm3 is calculated. The non-equilibrium properties of the plasma in the active zone and in the flowing afterglow are investigated by determining both the rotational temperature and the vibrational temperature. The rota-tional temperature is a measure for the gas kinetic temperature and is evaluated using two methods: by fitting the experimental OH emission band with a simulated spectrum and by means of the Boltzmann plot method. In an argon discharge the gas temperature increases from 620 K to 1130 K when the water vapor concentration is increased from 0% to 0.76%. In the second part of this work the plasma jet is applied to the surface treatment of different samples made from polypropylene and polytetrafluoroethylene (PTFE). The influence of the plasma treatment on the surface properties is quantified by water contact angle measurements (WCA), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photo-electron spectroscopy (XPS). More specifically it is investigated to what extent additives increase the effectiveness of the plasma treatment. The plasma treatment of polypropylene results in a chemical modification of the surface, with a substantial amount of oxygen being incorporated into the surface. The amount of incorporated oxygen can be increased by adding a small amount (0.05%) of water vapor to argon. This finding is in qualitative agreement with the previously described behavior of the OH emission intensity as a function of the composition of the argon-water mixture. A last topic of investigation is the effectiveness of CO2 as an ad-ditive for the treatment of a PTFE surface with a DBD plasma jet in argon. The active species in the plasma are identified by OES. The emission spectrum of an argon discharge contains lines from argon and atomic oxygen, and bands from N2 and OH. The spectrum of the discharge in a mixture of argon and CO2 reveals the additional pres-ence of radicals with CO groups. From the characterization of the surface it follows that a treatment time shorter than twenty seconds induces a small decrease of the contact angle. XPS-spectra show that this is a result of the incorporation of a small amount of oxygen. For longer treatment times the contact angle again increases, due to the breaking of polymer chains and the formation of a larger amount of oligomeric segments on the PTFE surface. These surface degrada-tion processes impede a substantial increase of the hydrophilicity of PTFE. The DBD plasma jet that has been designed and investigated in the frame of this work, is a useful tool for studying both the properties of a cold atmospheric plasma and the interactions of this plasma with a polymer surface.
Please use this url to cite or link to this publication:
author
promoter
UGent and UGent
organization
alternative title
Plasmastraal opgewekt door een diëlektrische-barrière-ontlading voor de opervlaktebehandeling van polymeren
year
type
dissertation (monograph)
subject
keyword
optical emission, spectroscopy, gas temperature measurements, atmospheric pressure plasma jet, OH radicals, polytetrafluoroethylene, OH rotational tem-perature, AFM and SEM, XPS, surface modification, contact angle measurements
pages
XXVIII, 135 pages
publisher
Ghent University. Faculty of Engineering and Architecture
place of publication
Ghent, Belgium
defense location
Gent : Faculteit Ingenieurswetenschappen (Jozef Plateauzaal)
defense date
2012-01-27 16:00
ISBN
9789085784807
language
English
UGent publication?
yes
classification
D1
copyright statement
I have retained and own the full copyright for this publication
id
2019045
handle
http://hdl.handle.net/1854/LU-2019045
date created
2012-02-03 15:28:15
date last changed
2012-02-06 08:45:29
@phdthesis{2019045,
  abstract     = {Plasma jets are sources of non-thermal or {\textquotedblleft}cold{\textquotedblright} plasma that are increasingly used in several applications in materials technology and, more recently, in medicine. A key advantage of plasma jet systems is the fact that the available plasma volume is not limited to the gap between the electrodes. By means of a gas \unmatched{fb02}ow through the system, a {\textquotedblleft}\unmatched{fb02}owing afterglow{\textquotedblright} is created that is brought in contact with the surface to be treated. The dimensions of the \unmatched{fb02}owing afterglow region can be set by the choice of gas, \unmatched{fb02}ow rate and geometry, which enables treating complex three-dimensional surfaces such as porous polymers and dental cavities. As the gas temperature in the afterglow region is relatively low, plasma jets are typically used in the surface treatment of heat-sensitive materials that can be exposed without the risk of thermal damage to the radicals (OH, O, ), metastables and UV-photons that are produced in the plasma. Although the under-standing of the detailed mechanisms is still incomplete, it can be stated in general terms that the mentioned {\textquotedblleft}active species{\textquotedblright} are responsible for initiating the surface processes that lead to the envisaged plasma effect. It follows that when designing a plasma jet, a high \unmatched{fb02}ux of active species is targeted. One way of achieving that is by a proper choice of the gas mixture. In the literature the in\unmatched{fb02}uence of adding small amounts of oxygen (O2 ) to the gas mixture is well documented. In this work two alternative additives, water vapor and CO2 are investigated.
The \unmatched{fb01}rst part of the thesis is a detailed characterization of a dielectric barrier discharge (DBD) plasma jet in argon, helium and mixtures of these gases with water vapor. The electrical properties of the plasma, i.e. the voltage-current characteristics and the power dissipated in the discharge, are determined from the voltage and current waveforms and from charge-voltage Lissajous \unmatched{fb01}gures respectively.
The results indicate that when water vapor is added at constant ap-plied voltage, the power coupled to the discharge decreases. In ad-dition, in water-containing discharges the current waveform features an asymmetry between the negative and positive half cycle of the applied voltage. The length of the \unmatched{fb02}owing afterglow of the plasma jet is pho-tographed under different experimental conditions (gas \unmatched{fb02}ow, mixture composition, applied voltage). For a helium discharge the length of the afterglow region \unmatched{fb01}rst increases from 21 mm to 54 mm when the gas \unmatched{fb02}ow rate is increased from 0.695 slm to 3.4 slm. Upon a fur-ther increase of the gas \unmatched{fb02}ow rate to 5 slm, the afterglow shrinks to 43 mm, as a result of the transition of the \unmatched{fb02}ow regime from laminar to turbulent. Adding water vapor shortens the afterglow. The same trends are observed for argon. It is evidenced that the \unmatched{fb02}owing after-glow of a plasma jet in helium consists of plasma bullets and that this type of plasma is sustained by photo-ionization processes. There is no difference in the plasma dynamics between the positive and neg-ative half cycle of the applied voltage. The propagation velocity of a plasma bullet is not in\unmatched{fb02}uenced by the addition of water vapor in the 0.05\%-0.76\% concentration range.
Time-resolved optical emission spectroscopy (OES) is applied to both the active plasma zone and the afterglow region. Atomic lines and molecular bands are identi\unmatched{fb01}ed, yielding a qualitative picture of the plasma composition. Important emission bands, including OH, are mapped with a spatial resolution of 1 mm. It is observed that when a small amount (0.5\%) of water vapor is added to a pure argon discharge, the intensity of the OH-emission is increased. At higher water vapor concentrations the intensity drops. This can be explained by the quenching of OH-radicals by collisions with water molecules and by a decrease of the electron temperature as a result of inelastic collisions between electrons and molecular additives (i.c. water). The latter effect also explains the intensity drop of the atomic lines when water vapor is added to the mixture. A good estimate of the electron temperature can be obtained by measuring the excitation temperature of argon. The latter amounts to 0.95 eV in pure argon and is con\unmatched{fb01}rmed to decrease through the addition of water. From the current density a value for the electron density of 1.5E13 cm3 is calculated.
The non-equilibrium properties of the plasma in the active zone and in the \unmatched{fb02}owing afterglow are investigated by determining both the rotational temperature and the vibrational temperature. The rota-tional temperature is a measure for the gas kinetic temperature and is evaluated using two methods: by \unmatched{fb01}tting the experimental OH emission band with a simulated spectrum and by means of the Boltzmann plot method. In an argon discharge the gas temperature increases from 620 K to 1130 K when the water vapor concentration is increased from 0\% to 0.76\%.
In the second part of this work the plasma jet is applied to the surface treatment of different samples made from polypropylene and polytetra\unmatched{fb02}uoroethylene (PTFE). The in\unmatched{fb02}uence of the plasma treatment on the surface properties is quanti\unmatched{fb01}ed by water contact angle measurements (WCA), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photo-electron spectroscopy (XPS). More speci\unmatched{fb01}cally it is investigated to what extent additives increase the effectiveness of the plasma treatment. The plasma treatment of polypropylene results in a chemical modi\unmatched{fb01}cation of the surface, with a substantial amount of oxygen being incorporated into the surface. The amount of incorporated oxygen can be increased by adding a small amount (0.05\%) of water vapor to argon. This \unmatched{fb01}nding is in qualitative agreement with the previously described behavior of the OH emission intensity as a function of the composition of the argon-water mixture.
A last topic of investigation is the effectiveness of CO2 as an ad-ditive for the treatment of a PTFE surface with a DBD plasma jet in argon. The active species in the plasma are identi\unmatched{fb01}ed by OES. The emission spectrum of an argon discharge contains lines from argon and atomic oxygen, and bands from N2 and OH. The spectrum of the discharge in a mixture of argon and CO2 reveals the additional pres-ence of radicals with CO groups. From the characterization of the surface it follows that a treatment time shorter than twenty seconds induces a small decrease of the contact angle. XPS-spectra show that this is a result of the incorporation of a small amount of oxygen. For longer treatment times the contact angle again increases, due to the breaking of polymer chains and the formation of a larger amount of oligomeric segments on the PTFE surface. These surface degrada-tion processes impede a substantial increase of the hydrophilicity of PTFE. The DBD plasma jet that has been designed and investigated in the frame of this work, is a useful tool for studying both the properties of a cold atmospheric plasma and the interactions of this plasma with a polymer surface.},
  author       = {Sarani, Abdollah},
  isbn         = {9789085784807},
  keyword      = {optical emission,spectroscopy,gas temperature measurements,atmospheric pressure plasma jet,OH radicals,polytetra\unmatched{fb02}uoroethylene,OH rotational tem-perature,AFM and SEM,XPS,surface modi\unmatched{fb01}cation,contact angle measurements},
  language     = {eng},
  pages        = {XXVIII, 135},
  publisher    = {Ghent University. Faculty of Engineering and Architecture},
  school       = {Ghent University},
  title        = {Dielectric barrier discharge plasma jet for the surface treatment of polymeric substrates},
  year         = {2012},
}

Chicago
Sarani, Abdollah. 2012. “Dielectric Barrier Discharge Plasma Jet for the Surface Treatment of Polymeric Substrates”. Ghent, Belgium: Ghent University. Faculty of Engineering and Architecture.
APA
Sarani, A. (2012). Dielectric barrier discharge plasma jet for the surface treatment of polymeric substrates. Ghent University. Faculty of Engineering and Architecture, Ghent, Belgium.
Vancouver
1.
Sarani A. Dielectric barrier discharge plasma jet for the surface treatment of polymeric substrates. [Ghent, Belgium]: Ghent University. Faculty of Engineering and Architecture; 2012.
MLA
Sarani, Abdollah. “Dielectric Barrier Discharge Plasma Jet for the Surface Treatment of Polymeric Substrates.” 2012 : n. pag. Print.