Dirk Hegemann

Plasma treatment of polymers for surface and adhesion improvement

Abstract Different plasma treatments in a rf discharge of Ar, He, or N 2 are used to etch, cross-link, and activate polymers like PC, PP, EPDM, PE, PS, PET and PMMA. Due to the numerous ways a plasma interacts with the polymer surface, the gas type and the plasma conditions must be adjusted on the polymer type to minimize degradation and aging effects. Wetting and friction properties of polymers can be improved by a simple plasma treatment, demonstrated on PC and EPDM, respectively. However, the deposition of ultra-thin layers by plasma enables the adjustment of wetting properties, using siloxane-based or fluorocarbon films, and further reduction of the friction coefficient, applying siloxane or a-C:H coatings. Nevertheless, the adhesion of plasma-deposited coatings should be regarded, which can be enhanced by depositing a graded layer.

Plasma functionalization of polypropylene with acrylic acid

Tailor-made surfaces of polymer materials are required, e.g. for the improvement of their printability or adhesion and for many applications concerning medical equipment or life science products. Polypropylene (PP) is one of the few common polymers widely used in technical applications. But often the surface chemistry has to be modified by introducing special chemical functionalities like carboxylic groups or by coating the PP with thin films, e.g. with poly(acrylic acid). The radio frequency plasma technique was used for functionalization and coating of PP by using acrylic acid as monomer gas. After optimization of the plasma parameters, high concentration of carboxylic groups as well as solvent-stable thin films with good adhesion to the PP substrates could be obtained. Characterization of the plasma-modified PP substrates was performed by using the captive bubble method, ESCA (Electron Spectroscopy for Chemical Analysis) and fluorescence spectroscopy in combination with derivatization techniques and FTIR (Fourier Transformation Infrared Spectroscopy). The thickness of the polymer films was analyzed by AFM (Atomic Force Microscopy).

Plasma grafting : a method to obtain monofunctional surfaces

Tailor-made surfaces are needed for many applications concerning medical equipment or life-science polymer material. The polymers should only reveal one type of functionality (carboxyl groups), homogeneously distributed with a defined density over their total surface. Many attempts have been made to obtain those monofunctional surfaces on polymers via plasma treatments. The commonly used plasma treatment (with oxygen) results in a host of different functionalities often with a low stability. The latter is caused by damaging processes also occuring during the plasma treatment (fragmentation by charged particle bombardment or radiation damage). Therefore, it is desirable to minimize or, if possible, to completely avoid these effects. In principle, two kinds of strategies are used: first, minimising the applied energy (e.g. use of low power or pulsed powered plasmas) and minimising the kind and density of damaging particles during treatment in the plasma; second, separating substrate functionalisation from plasma in space (down stream) or in time (grafting). Both methods lead to a more homogenous distribution of functionalities and a better retention of the precursor structure. The aim of this contribution is to give a comparison of several approaches to produce homogeneously finished polymer surfaces. Attention is focused on plasma grafting. As substrate, polypropylene is chosen because of its widespread applications and its simple chemical composition, which makes observation of grafting yields easy to analyse. As functional groups, hydroxyl, carboxyl and epoxy groups are chosen. These are introduced by direct plasma treatment and also by grafting methods. Functional groups are observed by ESCA and IR measurements. The influence of the gases used to activate the polymer surface for grafting carboxyl groups is also discussed.

Deposition of SiOx films from O2/HMDSO plasmas

The variation of O2/HMDSO ratio in an r.f. plasma was carried out to optimize SiOx film properties and deposition rates for the coating of polymers. A ratio exceeding 6:1 yields scratch resistant, quartz-like films with low carbon contents. The chemical composition was analyzed to SiO1.8C0.3 by XPS. A compact plasma reactor was developed generating homogeneous plasmas and depositions over large areas (up to 380×290 mm2). Furthermore, the total coverage of three-dimensional formed parts was managed with good adhesion of SiOx films to polymers like PC and PBT to reduce wear.

Plasma Treatment of Polymers to Generate Stable, Hydrophobic Surfaces

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.

Adsorption of protein on plasma-polysiloxane layers of different surface energies

Abstract We have studied the influence of the surface energy of the substrate to the adsorption of the protein albumin from an aqueous solution. Therefore, coatings of polysiloxane of different surface energies were deposited in a plasma process and used as substrates. The adsorption of the protein albumin on this layers was studied by infrared spectroscopy and wetting analysis, i.e. the determination of the change of the interfacial tension at the solid–liquid interface due to the adsorption by combining contact angle and surface tension data. We found that the amount of adsorbed protein decreased with increasing surface energy of the substrate. The interfacial energy of the solid–liquid interface increases for substrates of high surface energy, which indicates that the decrease of interfacial energy is not the only driving force of this adsorption but also an expulsion of the proteins from the solution takes place (hydrophobic interaction).

EVALUATION OF DEPOSITION CONDITIONS TO DESIGN PLASMA COATINGS LIKE SIOX AND A-C:H ON POLYMERS

Abstract RF excited gas discharges of hexamethyldisiloxane (HMDSO) with and without oxygen and methane (CH4) without further carrier gas were examined to identify reaction parameters determining deposition rate and film properties of SiOx and a-C:H films, respectively. This evaluation is supported by the used reactor type which enables well-defined deposition conditions. Both symmetrical and asymmetrical electrode configurations are investigated. It is found that the deposition rate depends on the reaction parameter power input per gas flow where the gas flows of monomer and reactive carrier gases are added. Thus, O2 in conjunction with HMDSO can be considered as a film-forming gas. Even in asymmetrical discharges the concept of the reaction parameter W/F holds, as long as the increasing ion bombardment allows a continuous film growth. While W/F controls the chemical composition of the films, the potential drop across the plasma sheath influences the mechanical film properties. With these findings SiOx and a-C:H plasma coatings can be designed.

Influence of single-source precursors on PACVD-derived boron carbonitride thin films

This work investigates the influence of the B/C/N containing single-source precursors pyridine-borane (PB) and triazaborabicyclodecane (TBBD) on the chemical composition of boron carbonitride thin films. The films are deposited via a PACVD process, activated by 13.56 MHz radio frequency (r.f.). N2, Ar and He serve as carrier gases. The chemical compositions of the coatings are analyzed by auger electron spectroscopy and the fragmentation of TBBD by mass spectrometry. It becomes evident, that from a certain bias voltage, the self-bias in capacitively r.f. electrical discharges mainly influences the chemical composition of the BCN films independent of the used precursor.

Plasmabehandlung von Polymeren für biologische und medizinische Anwendungen

Der Artikel beschreibt das Potenzial von Niederdruckplasmen zur Anpassung der Oberflacheneigenschaften polymerer Werkstoffe an Bedingungen in Biologie und Medizintechnik. Nach einer kurzen Diskussion der Anforderungen an die Materialien und Beschreibung des Verfahrens wird an den Beispielen Plasmasterilisation, Beeinflussung der Prote inadsorption und Optimierung des Zellwachstums durch Oberflachenbehandlung der gegenwartige Entwicklungsstand dargestellt. n n n nPlasma Treatment of Polymers for medical and biological Applications n n n nThe paper describes the possibilities of a low pressure plasma treatment of polymer surfaces to adapt the surface properties to the demands in biology and medical engineering. After a summary of these demands and a description of the plasma process we show examples of sterilization by plasma, controlling of the protein adsorption by plasma coating and the improvement of cell growth on the polymer by a plasma treatment.

Charakterisierung plasma- modifizierter Polymere mittels SSXPS und XPS-Imaging. SSXPS und XPS-Imaging on plasmamodified polymers

Polymere Werkstoffe mit masgeschneiderten Oberflacheneigenschaften stehen seit vielen Jahren im Mittelpunkt der Forschung und Entwicklung. Eine haufig genutzte Technik zur Funktionalisierung und Beschichtung von Polymeren stellt dabei die Plasmatechnik dar. Ausschlaggebend dafur ist, dass mit diesem Verfahren Kunststoffe, die nasschemisch nur schwer oder gar nicht zu modifizieren sind, ohne Beeintrachtigung der gewunschten Volumeneigenschaften gezielt an der Oberflache funktionalisiert bzw. haftfest beschichtet werden konnen. Der Einsatz technischer Polymere im Bereich Medizintechnik, life-science und Biotechnologie fordert zudem monofunktionalisierte und strukturierte Oberflachen, die ebenfalls durch geeignete Prozesfuhrung generiert werden konnen. Zur Charakterisierung dieser masgeschneiderten Oberflachen stehen verschiedene analytische Verfahren zur Verfugung. Neben AFM und FTIR kommt hier vor allem auch die Rontgenfotoelektronenspektroskopie (XPS, ESCA) zum Einsatz. n n n nFor many years now, engineering polymers with tailor-made surface properties are of widespread interest. One of the most commonly used technique to functionalize or coat polymers is the plasma technique. One of the most important reasons is the possibility to tailor the surface properties without alteration of the versatile bulk properties. Many engineering polymers applied in medical equipment, life-science, and biotechnological purposes demand monofunctionalized and structured surfaces. Those can be obtained with appropriate process parameters. n n n nThe characterization of such tailored surfaces as well as of surfaces with structured functionality can be performed by several analytical tech niques. Besides AFM and FTIR the photoelectron spectroscopy (XPS, ESCA) is used.