R. Barni

Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment

The surface chemical and physical modifications of poly(ethylene terephthalate) (PET) fibers induced by radiofrequency air plasma treatments were correlated with the characteristics of the discharge parameters and the chemical composition of the plasma itself, to identify the plasma-induced surface processes prevailing under different operating conditions. Treated polymer surfaces were characterized by water droplet absorption time measurements and XPS analysis, as a function of the aging time in different media, and by AFM analysis. They exhibited a remarkable increase in hydrophilicity, accompanied by extensive etching and by the implantation of both oxygen- and nitrogen-containing polar groups. Etching was mainly a consequence of ion bombardment, yielding low molecular weight, water soluble oxidation products, while surface chemical modifications were mainly due to the action of neutral species on the plasma-activated polymer surface.

Characterisation of poly(ethylene terephthalate) and cotton fibres after cold SF6 plasma treatment

Poly(ethylene terephthalate) (PET) and cotton fibres, treated in an appositely set up RF SF6 plasma reactor under different operating conditions, were characterised by XPS, EPR, DSC, XRD, ATR analyses, water contact angle and water droplet roll-off angle measurements. The ageing of plasma-treated samples was also investigated under different post-treatment conditions. Plasma treatment led to efficient implantation of fluorine atoms on the surface of both polymers; this resulted in water repellence without altering the bulk properties of the polymers. The radical species formed in the plasma-activated polymer surface were involved in its fluorination and in the subsequent uptake of atmospheric oxygen. Surface reorganisation of polymer segments, tending to reduce the interfacial energy between the polymer and the phase in contact with it, induced the surface modifications observed under ageing and in samples plasma-treated several times. An increase in the depth of the fluorinated layer, leading to outstanding stable hydrorepellence, was achieved by repeated SF6 plasma treatments, followed by surface rearrangements favoured by swelling.

A SF6 RF plasma reactor for research on textile treatment

This research concerns the development of a SF6 RF discharge at low pressure in a small reactor for industrial applications. The plasma is produced in the pressure range 0.05-1 mbar by a RF supply. The pumping system sustains a flowrate of about 50 cm s-1, with residence time in the discharge of about 0.2 s at a pressure of 0.1 mbar. The discharge parameters were measured at a low operation power. Measurements were performed by means of movable electrostatic probes and a photodiode. Particular care in the analysis of the data proved to be necessary due to the presence of a substantial amount of negative ions. The reactor has been employed for textile treatment in order to modify the surface properties of the fibres. Favourable operating conditions leading to an improved hydrophobicity of the textiles were achieved.

Characterization of plasma processing for polymers

Abstract Some selective plasma treatments are described, aiming at modifying specific surface properties of textile polymeric materials, such as their hydrorepellence and dyeability. The prevailing plasma–polymer interactions were identified by correlating the physico-chemical modification of treated polymer surfaces to the characteristics of the plasma sources.

Modification of the PTFE wettability by oxygen plasma treatments: influence of the operating parameters and investigation of the ageing behaviour

The effects of O2 plasma treatments on poly(tetrafluoroethylene) (PTFE) sheets are deeply investigated. The chemical modifications owing to the plasma treatment are studied by means of attenuated total reflectance Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy (XPS), while the surface topography of the plasma-treated samples is assessed by atomic force microscopy (AFM) analyses. Results are correlated with the wettability characteristics of the plasma-modified PTFE sheets, which is studied by means of contact angle (CA) and roll-off angle measurements. Finally, the ageing of the plasma-treated surfaces is investigated. The RF power input and the treatment time influence the chemical/morphological characteristics and the wettability of the plasma-treated samples. PTFE samples treated at low-power input (up to 50 W) are more hydrophilic than the untreated one. The XPS analysis reveals that OH functionalization of the outermost layers of polymeric chains decreases by increasing the RF power input, and dynamic CAs gradually increase. The AFM analysis shows a strong increase in the surface roughness as a consequence of the differential etching of the PTFE surface for samples treated at power inputs ≥100 W, with formation of globular structures. XPS analyses of the aged samples reveal a post-plasma oxidation. However, the dynamic CAs measured after 30 days are greater than those measured immediately after treatment, and for power input ≥100 W the aged plasma-treated PTFE surfaces are super-hydrophobic. This result suggests a decrease in the surface density of the exposed hydroxyl groups during ageing time, as a consequence of surface adaptation.

Characterization of the streamer regime in dielectric barrier discharges

The streamer regime of a dielectric barrier discharge device is studied by performing a detailed statistical analysis of current-voltage measurements in air. A wide bandwidth Rogowski coil, designed to work down to the nanoseconds time scale, is used to record the discharge current. The temporal structure of the latter is identified and characterized by its probability density distribution as a function of the applied voltage. The results suggest the existence of two discharge regimes, separated by a well defined voltage threshold, reflecting the different behaviors of the microdischarges. The autocorrelations of the discharge signal are evaluated as a function of the applied voltage, indicating the presence of strong correlations at short-time scales (up to the order of 102 ns) and residual correlations at longer times. The latter are shown to be due to the nonstationarity of the discharge process.

Chemical kinetics simulation for atmospheric pressure air plasmas in a streamer regime

We present a theoretical analysis of air discharges at high pressure which operates in a streamer regime. The aim is to provide a simplified framework to study the chemical kinetics in the discharge gas phase. We have tried to model the characteristics of actually existing devices, such as dc and ac discharges in sharp point-plane, point to point, or coaxial cylinder configuration, dielectric barrier discharges. Within such scheme it is possible to perform numerical simulations in order to predict the evolution of the gas-phase chemical composition. As a first application we have chosen to simulate the chemical kinetics induced in a single isolated streamer and the results of this approach are examined within this paper. In particular, we have studied the detailed temporal evolution of the discharge gas phase using reference values for the simulation free parameters. The interplay between different time scale processes is shown. The effect due to charged as well as metastable species on chemical kinetics ...

Optical and electrical characterization of a surface dielectric barrier discharge plasma actuator

An experimental characterization of the properties of asymmetric surface dielectric barrier discharges used as plasma actuators was performed. Optical emission spectroscopy was used to measure the radiated power and some plasma parameters such as the electron and vibrational temperature. Electrical characterization of the discharge was executed by recording individual current pulses with high temporal resolution, and collecting a large dataset of these events. Statistical analysis performed on them allowed one to correlate microdischarge (MD) properties with the voltage phase and to spot differences arising from the actual breakdown mechanism in such asymmetric configurations. In particular, the asymmetry between the two different half-cycles of the discharges was characterized, and it was found that it directly influences plasma actuator efficiency. Differences arising in the multiplicity, amplitude and temporal duration of the MDs were investigated. Some effects connected with the dielectric material and high voltage supply properties were evaluated and correlated with the induced velocity provided by the plasma actuators.

Experimental study and simulations of electronegative discharges at low pressure

We have performed an experimental and theoretical analysis of the rf discharges at low pressure in a small reactor for industrial applications. In particular, we focused on the plasma produced from electronegative gases (air and SF6) in the low pressure range between 0.05 and 1 mbar. The device pumping system sustains a flow velocity of about 50 cm/s, with residence time in the discharge of about 0.2 s, at P=0.1 mbar. The discharge parameters have been measured by means of movable electrostatic probes and lies in the range ne≅107–109 cm−3, ni≅1010–1012 cm−3, and Te≅5–10 eV. Particular care in the analysis of the data proved necessary due to the presence of a substantial amount of negative ions. On the other hand, we have performed simulations of the discharge composition through the implementation of a suitable numerical model of the chemical kinetics in the device. On the basis of the experiments, a comparison of charged species production with those predicted by the theory was performed and a more reali...

Experimental study of outgassing from textile materials

We have performed a careful analysis of the pumpdown curves measured in our vacuum reactors for radio frequency plasma treatment of materials at low pressure. The characterization of the device pumping behavior both in the viscous and in the molecular regimes makes it possible to study the outgassing flow rates of several textile materials. Estimates of the relevant flow rates in different pressure ranges and measurements of the flow composition have been obtained. Such data can be interpreted in the framework of simplified desorption models and could be used in order to design the pumping system needed for vacuum treatment of such materials, both in batch and in continuously fed-in processes. In particular direct application of existing cold plasma treatments, such as chemical vapor deposition, plasma etching, activation, and grafting, at an industrial scale can be foreseen in view of our data, which can be straightforwardly used to estimate the costs of the required vacuum technology.