Showing papers in "Plasmas and Polymers in 2001"

Plasma sterilization: a review of parameters, mechanisms and limitations

Abstract: Low-temperature plasma is a promising method for destroying microorganisms, an alternative to “conventional” methods which have numerous drawbacks. Several plasma-based sterilization technologies are presently under development, but their mechanisms of action are still incompletely understood. Since more than five years, we have investigated the effects of plasma on microorganisms (killing efficacy, and related mechanisms), as well as on the materials being sterilized. This article reports some important observations made during this work, using the commercialized so-called “plasma sterilizers” and “real” low-pressure plasma systems. The mechanism of etching (volatilization) of microorganisms by plasma that we have observed, leads us to believe that plasma may constitute a powerful solution to the clinical problems of deactivating also prions and endotoxins. However, plasma effectiveness is influenced by numerous experimental parameters, which we review here. This inherent complexity, and the weak penetrating power of plasma species, that severely limits plasma effectiveness in the presence of organic residues, packaging material, or complex geometries, are the main limitations of plasma sterilization.

Overview of Atmospheric Pressure Discharges Producing Nonthermal Plasma

Abstract: Recently, much attention has been paid to gas discharges producing nonthermal plasma because of many potential benefits in industrial applications. Historically, past work focused on Dielectric Barrier (silent) Discharges (DBD) and pulse-periodical corona discharges. Recently, a number of new different discharge techniques succeeded in producing stable gas discharge at atmospheric pressure. Among these are repetitively pulsed glow discharge; continuous glow discharge in a gas flow; hollow-cathode atmospheric pressure discharge; RF and microwave (MW) discharges. Several new variants of the DBD have been demonstrated over a rather wide range of frequencies. All these forms of gas discharge are characterized by a strong nonequilibrium plasma state. We attempt to classify these discharges with respect to their properties, and an overview of possible applications is made. Conditions for the existence of homogenous and filamentary forms of each of the discharge types are discussed.

The Role of Dielectric Barrier Discharge Atmosphere and Physics on Polypropylene Surface Treatment

Abstract: Dielectric barrier discharge (DBD) is the discharge involved in corona treatment, widely used in industry to increase the wettability or the adhesion of polymer films or fibers. Usually DBD's are filamentary discharges but recently a homogeneous glow DBD has been obtained. The aim of this paper is to compare polypropylene surface transformations realized with filamentary and glow DBD in different atmospheres (He, N2, N2 + O2 mixtures) and to determine the relative influence of both the discharge regime and the gas nature, on the polypropylene surface transformations. From wettability and XPS results it is shown that the discharge regime can have a significant effect on the surface transformations, because it changes both the ratio of electrons to gas metastables, and the space distribution of the plasma active species. This last parameter is important at atmospheric pressure because the mean free paths are short (∼μm). These two points explain why in He, polypropylene wettability increase is greater by a glow DBD than by a filamentary DBD. In N2, no significant effect of the discharge regime is observed because electrons and metastables lead to the same active species throughout the gas bulk. The specificity of a DBD in N2 atmosphere compared to an atmosphere containing oxygen is that it allows very extensive surface transformations and a greater increase of the polypropylene surface wettability. Indeed, even in low concentration and independently of the discharge regime, when O2 is present in the plasma gas, it controls the surface chemistry and degradation occurs.

Deposition Process Based on Organosilicon Precursors in Dielectric Barrier Discharges at Atmospheric Pressure—A Comparison

Abstract: Dielectric barrier discharges (DBD) at atmospheric pressure are presented as a tool to create organosilicon deposits on technical planar aluminium substrates (up to 15 × 8 cm2) by admixing small amounts of hexamethyldisiloxane (HMDSO) and tetraethoxysilane (TEOS) to the carrier gas of the discharges. Using barrier materials of different specific capacities (2.6 × 104 and 3.2 pF/cm2) in two electrode arrangements operated at less than 1 W, the influence of the filament properties on the deposition is studied. In comparison to these arrangements, a third electrode setup with a barrier of the specific capacity of 2.9 pF/cm2 is operated at approximately 50 W to study the influence of the specific energy of the plasma (energy per molecule) on the deposition process. The plasma chemical process was studied qualitatively by Gas Chromatography, and properties of the plasma-treated substrates were examined by means of X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared (FTIR) spectroscopy, as well as visually.

Ultrathin Silicon-Compound Barrier Coatings for Polymeric Packaging Materials: An Industrial Perspective

Abstract: Growing demands for increased shelf-life of food products and chemical inertia of the contact surfaces have stimulated development of polymers with improved high-barrier properties. Our objectives in this article are (1) to describe experimental results on Plasma-enhanced chemical vapor deposition (PECVD) and its importance to produce thin layers of inorganic glassy barrier materials for food, pharmaceutical, and organic display applications; (2) despite the thereby greatly enhanced quality of film or rigid packaging material, some residual coating defects result in less-than-perfect gas, moisture and aroma barriers: an innovative technique based on reactive ion etching (RIE) in oxygen plasma is also presented, along with a staining method, which render even sub-μm coating defects visible. Data are shown for oxygen transmission rate on virgin and defective coatings, and the industrial context and applications are presented.

Plasma Treatment of Polymers to Generate Stable, Hydrophobic Surfaces

Abstract: Tailoring of polymers for multifaceted applications is an increasing field, whereby most often the surface properties must be adjusted. Therefore, the coating of common polymers by plasma polymerization is a promising way to modify the surface and meet the demands. Beside the tuning of the required surface properties, good adhesion and stability of the films is essential. This work investigates the plasma deposition of pp-HMDSO films on PC and PC/ABS to generate stable, hydrophobic surfaces. By examining the plasma conditions—deposition rate, energy range, and surface topography—ultrathin, stable films with advancing contact angles up to 110° and receding angles exceeding 90° can be designed. Storage of the siloxane films for 1 year in air at ambient conditions exhibits almost no aging. Thus, these films are superior to fluorocarbon films deposited for comparison.

RF Plasma Deposition of PEO-Like Films: Diagnostics and Process Control

Abstract: Organic thin films deposited by means of radio-frequency glow discharges fed with Triglyme vapors have been investigated to explore the feasibility for deposition of organic thin films with polyethylene oxide-like features. The film chemical composition has been analyzed by means of X-ray Photoelectron Spectroscopy and FT Infrared Absorption Spectroscopy. Plasma phase diagnostics has been accomplished by means of Optical Emission Spectroscopy. It is shown that the surface density of ether carbon, which is considered the marker of the content of ethylene oxide units in the coating, decreases as the power input is increased. It is also shown that the retention of monomer structure in the film can be easily controlled in situ by actinometric optical emission spectroscopy.

Surface Modifications Produced by N2 and O2 RF Plasma Treatment on a Synthetic Vulcanized Styrene-Butadiene Rubber

Abstract: A low-pressure gas RF plasma-treatment has been used to improve the adhesion of a synthetic vulcanized rubber to polyurethane adhesive as an environmentally friendly alternative surface treatment to the conventional chemical treatments. A sulfur vulcanized styrene-butadiene rubber (R2) containing a noticeable amount of zinc stearate and paraffin wax (both providing a lack of adhesion) in its formulation was used. Two different gases (oxygen and nitrogen) were used to generate the RF plasma, which was performed at 50 Watt for 1–15 min. The modifications produced on the R2 rubber surface by the RF plasma treatments were assessed by using advancing and receding contact angle measurements, ATR-IR spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Force Microscopy (SFM), and Scanning Electron Microscopy (SEM). Adhesion evaluation was obtained from T-peel tests of joints produced between plasma treated R2 rubber and a polyurethane adhesive. The plasma treatment produced a decrease in advancing and receding contact angle values on R2 rubber, irrespective to the gas used to generate the RF plasma. The treatment with RF plasma produced the partial removal of hydrocarbon moieties from the rubber surface and the generation of oxygen moieties. An increase in surface roughness was also produced. The degree of oxidation and the amount of hydrocarbon-rich layer removed from the R2 rubber surface was more important by treating with oxygen plasma. The treatment of rubber in oxygen plasma for 1 minute was enough to noticeably increase adhesion of R2 rubber to polyurethane adhesive. However, an extended treatment (15 min.) was needed when nitrogen plasma was applied to R2 rubber. The loci of failure in the joints produced between the plasma treated R2 rubber and the polyurethane adhesive was assessed by using ATR-IR spectroscopy. A mixed failure (partially adhesional and partially cohesive failure in the rubber) in the joints produced with plasma treated R2 rubber joints was always obtained.

A plasma surface treatment of polyester textile fabrics used for reinforcement of car tires

Abstract: Polyester tire cord surfaces have been modified by plasma at low temperature and atmospheric pressure The surface treatment has been executed by various nonequilibrium discharges, namely by barrier discharge, atmospheric pressure glow discharge and gliding arc The polymeric multicord sewing threads treated by this procedure have been used in the same form as in industry, ie, with the protecting oil films on their surface The surface properties have been investigated by electron spin resonance spectroscopy and by measuring their contact angle with various liquids; partially the zeta potential measurements have been used, too Further tests have been done at an industrial testing impregnation line using the common technology and conditions, on both plasma treated and untreated fibers Finally, the standard H-tests and peel-tests have been used to characterize the fiber adhesion to usual testing rubbers

Pulsed Plasma Polymerization of Cyclic Ethers: Production of Biologically Nonfouling Surfaces

Abstract: Polymerization of low molecular weight cyclic ethers was investigated under pulsed plasma conditions Film formation conditions were adjusted to optimize retention of ethylene oxide (EO) content of the monomers in the resultant plasma generated polymers To a large extent this goal was achieved with the 12-crown-4 and 15-crown-5 monomers, but not with dioxane Films obtained from the 12-crown-4 monomer under ultra low power inputs are shown to be highly resistant to protein adsorption, while exhibiting good chemical compositional stability and adhesion during prolonged immersion in aqueous solutions The dramatic differences observed in contrasting polymer film compositions from 12-crown-4 and dioxane are believed to arise from distinctive differences in the low electron impact fragmentation patterns of these two compounds, as discussed in this report

Plasma techniques for preparation of controlled drug release system

Abstract: Plasma-induced surface radicals formed on a variety of organic polymers have been studied by electron spin resonance (ESR), making it possible to provide a sound basis for future experimental design of polymer surface processing, i.e., plasma treatment. On the basis of the findings from such studies, several pharmaceutical applications in the field of drug engineering have been devised, which include preparation of double-compressed tablets for reservoir-type drug delivery system (DDS) of sustained- and delayed-release, and fabrication of functionalized composite powders applicable for matrix-type DDS by a mechanical application of plasma-irradiated polymer powder.

NH3 plasma treatment of PET for enhancing aluminium adhesion

Abstract: A NH3 plasma process has been studied for enhancing the adhesion of aluminum coatings on polyethyleneterephtalate (PET) films. According to our peel strength results, NH3 plasmas increase markedly the adhesion of aluminum on PET compared to O2 discharges, with a much shorter treatment time. A tentative model of nonhindered growth of Al-coating based on the Lewis basic character of the functionalities grafted by NH3 plasma is proposed for Al-polymer interactions, and for explaining the various steps in the process. The effects of power input and treatment time on the polymer surface chemistry and on the metal-polymer peel strength have been evaluated. Treatment times as short as 0.1 s at 100 W proved to be the best conditions in NH3 plasmas, for a significant increase in Al/PET adhesion, while longer treatments have a detrimental effect. This may explain why most authors have not discovered the benefits of NH3 plasmas for improving the adhesion of metals on PET, and have preferred O2 or air treatments. The relative basicity of PET grafted with N-containing functionalities has been measured by means of X-ray Photoelectron Spectroscopy (XPS) analysis of samples exposed to vapors of trichloromethane, a Lewis acid molecular probe. The Al/PET adhesion was evaluated by means of a 180° Peel Test.

Atomic oxygen concentration in a flowing post-discharge reactor

Abstract: Concentration of neutral oxygen atoms in the flowing post-discharge of a pure oxygen microwave discharge at different experimental conditions was determined with a nickel catalytic probe. The post-discharge reactor was setup for metal surface cleaning. It worked at the pressure between 20 and 100 Pa and at output power of the microwave plasma generator between 80 and 150 W. At those experimental conditions the O-atom density was found to be of the order of 1021 m−3. It increased both with increasing pressure and microwave power. The degree of dissociation of oxygen molecules, on the other hand, decreased with increasing pressure.

XPS Analysis of the Aging of Thin, Adhesion Promoting, Fluorocarbon Treatments of DC Plasma Polymers

Abstract: Plasma polymer treatment of aluminum alloys has recently been shown to improve adhesion of primer coatings, thereby reducing the corrosion of thusly protected panels to the levels afforded by conventional chromate conversion coating. One particular plasma polymer system, comprised of a ∼50 nm trimethylsilane DC plasma polymer capped by an ultrathin layer modified by DC hexafluoroethane plasma treatment, has shown tremendous adhesion increases to a wide variety of primers, yielding a coating that is virtually unremovable with conventional stripping applications. An application window was empirically deduced regarding this improved adhesion, indicating that the primer needed to be applied within 5 days of plasma treatment to display the tenacious adhesion to panels. In an effort to elucidate the differences between fresh and aged panels, an X-ray photoelectron spectroscopy (XPS) time study of this system was undertaken. Some direct correlation to this time frame was observed in the XPS data, indicating that a particular fluorocarbon structure in the films modified upon continued atmospheric exposure, rearranging the local bonding environment by introducing additional C—C bonding with an increase in oxygen incorporation.

Polyelectrolyte Adsorption on NH3-Plasma-Treated Poly(tetrafluoroethylene-co-hexafluoropropylene)

Abstract: A two-step procedure for a permanently hydrophilic surface modification of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP, fluorinated ethylene propylene) was studied. In the first step a cationic polymer surface was created by low-pressure ammonia plasma treatment introducing nitrogen-containing functional groups. Afterwards, the anionic poly(sodium 4-styrenesulfonate) was adsorbed onto the plasma-treated FEP surface. The adsorption was assumed to be controlled by ionic interactions. The modification effects and their long-term behavior were evaluated by means of water contact angle goniometry. Furthermore, electrokinetic measurements and X-ray photoelectron spectroscopy were used for surface characterization.

Application of Spray-Type Atmospheric Pressure Glow Plasma Reactor: Ashing of Organic Compounds II

Abstract: When plasma treatment is carried out in the after glow region of an electrical discharge, the decay rate and the density of the active species are very important factors for the treatment efficiency. They are known to depend on the linear gas flow rate (gas velocity) and on the residence time of the treatment gas in the discharge zone, respectively. In our previous study, we found that the spray-type atmospheric pressure glow plasma reactor with O2/He or O2/Ar mixture treatment gases had a satisfactory ashing rate of a solid organic compound (OFPR-800; a photoresist). However, the relationships between the gas velocity or the residence time and the ashing rate had not yet been examined. The present study showed clearly that the gas velocity influenced only the transit time, that is the time which the gas mixture took to progress from the slit nozzle to the sample surface, but it did not influence the generation of the active species. On the other hand, the generation rate of active species in the discharge zone was found to be strongly dependent on the residence time. The ashing rate was found to increase with increasing the residence time up to about 30 ms, beyond which it saturated. From optical emission spectroscopy measurements, the maximum ashing rate could be correlated with the emission intensities corresponding to He 3p3P-2s3S (388.8 nm) and O 3d5D-3p5P (926.5 nm) bands. These results are of practical interest.

ESR Study of Trimethylsilane Plasma Polymer Part II; Effect of Consecutive Treatments and Mixed Gases

Abstract: An ESR study has indicated that a second plasma treatment on plasma deposited films from trimethylsilane (TMS) monomer gas has the ability to modify the characteristics of the primary plasma polymer significantly in a favorable manner for many applications. The effect of the second plasma polymerization on the primary plasma polymer of TMS depends on the nature of the second monomer. Plasma of F-containing monomer, hexafluoroethane (HFE) and perfluoromethane (CF4), decreases the ESR signal of TMS and no detectable signal due to F-containing monomer was found. The decay rate of the signal decreased significantly. In contrast to this situation, CH4 plasma treatment yields an ESR signal that is a composite of that observed from TMS and CH4 films individually. The overall signal increased in this instance, but didn't show appreciable decay in 24 hr period. A second treatment by nonpolymer forming plasmas also decreased the ESR signal of TMS, and decreased the decay rate, indicating that the second gas plasma treatment yields a somewhat similar effect found with the HFE plasma treatment. Plasma polymerization of mixtures of TMS and nonpolymer-forming gases increased the ESR signal but decreased the decay rate, except in the case of oxygen. A mixture of (TMS + O2) behaved as a completely different monomer. No ESR signal was found in this system. The ESR analysis was supported by XPS data and an insight into the mechanisms occurring in these thin films are discussed.

Nanostructures, Microscale Technologies, and Plasma Deposited Films

Abstract: Richard Feynman’s 1960 talk, “There’s Plenty of Room at the Bottom” is often cited as the stimulus for the modern interest in nanotechnology and nanofabrication.(1) But, plasma scientists, “complaining” about thin films (nanometers to microns) coating the walls their reactors, have been doing unintentional nanofabrication even before Irving Langmuir coined the term, “plasma.” Another group involved with nanotechnology and microtechnology well before the words had cachet and funding is the molecular biologists—the protein “machines,” lipid bilayer membranes, ion channels and supramolecular structures they describe are all at the nanometer to micron scale. Thus, these technologies of small things are not so new as many perceive it to be. Still, it’s interesting to view plasmas in the context of modern nanotechnology and microtechnology and ask about opportunities and trends. A quick review of the literature relating to technology and fabrication at a scale between 1 and 10,000 nanometers reveals the following themes to be popular: