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- Allyl alcohol (3)
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Eingeladener Vortrag (wissenschaftliche Konferenzen)
- nein (15)
Polypropylen (PP) ist als Niedrigenergie-Polymer ohne nass- und plasmachemische Vorbehandlung weder haftfest zu beschichten noch verbundfest zu kleben. Typische Haft- bzw. Klebfestigkeiten liegen für die unbehandelte Polymeroberfläche bei maximal 0,5 MPa. Hinzu kommt, dass bei der Bestimmung von Haft- und Klebfestigkeiten der unmodifizierten Oberfläche mit großen Streuungen zu rechnen ist, die bezogen auf die Spannweite durchaus einem Faktor 5 entsprechen können. Generell besteht bei Prüfung der Haftfestigkeit mittels Stirnabzugsversuch und bei der Prüfung der Klebfestigkeit die Notwendigkeit eines statistisch validierten Herangehens. Hierzu sind zeitaufwändige Ein-Proben-Klebungen und -Prüfungen nur eingeschränkt geeignet. Zudem ist, insbesondere für geringe Haft- bzw. Klebfestigkeiten, ein querkraftfreies Prüfen Grundvoraussetzung. Im vorliegenden Beitrag werden verschiedenartige plasmagestützte Oberflächenfunktionalisierungen von PP sowie das Mehr-Proben-Konzept zur Klebung/Prüfung von bis zu acht Proben vorgestellt. Dazu werden ein BAM-Funktionsmuster einer Mehr-Proben-Kleb-Vorrichtung und eine kommerzielle analytische Zentrifuge zum Prüfen verwendet.
For simulation and acceleration of artificial polymer ageing, polyolefin foils were exposed to low-pressure Ar plasma. Plasma particle bombardment and irradiation induce C–C and C–H bond scissions by σ→σ* excitations on the surface and in near-surface layers. Consequently, radicals are generated. They react by recombination, cross-linking, metastable trapping of the radical site or formation of olefinic double bonds. The long-living and metastable trapped C-radicals as well as double bonds in polyolefins were immediately exposed to bromine vapour without breaking the vacuum after switching-off the plasma. These reactive sites rapidly react with the molecular bromine under formation of C–Br bonds. For 5 min of argon plasma exposure, the elemental concentration of bromine was 13% for polyethylene and 22% Br/C for polypropylene as analysed by X-ray photoelectron spectroscopy. Nevertheless, not all C radical sites have reacted with bromine. Later on, when the polyolefins brought in contact with ambient air, an additional post-plasma reaction of the remaining trapped radicals with oxygen was observed. The oxygen concentrations were lower after bromine gassing, thus repressing partially the post-plasma oxidation in the analysed layer (ca. 6 nm) by radical quenching. Such bromination took place either at the surface or in near-surface layers because the Attenuated Total Reflectance (ATR)-FTIR spectra (sampling depth ca. 2500 nm) did not show significant changes for argon plasma-treated PE foils with and without bromine vapour exposure. Further addition of bromine may also occur on C=C double bonds.
Multiple-sample approach: reliable ranking and validated statistics of bonding and adhesive strength
(2014)
Bonding strength of adhesive-bonded joints and adhesive strength of coatings have been exclusively determined in single-sample tests using a tensile testing machine. Necessarily, the single-sample approach was also applied to the corresponding bonding procedure. As a consequence, reliability and reproducibility of results were restricted. By applying the newly introduced centrifuge technology, the multiple-sample approach was realized for simultaneous tensile testing of up to eight samples under identical conditions without any disturbing shear-force effects. In order to introduce the multiple-sample approach into the bonding procedure, a hydraulic bonding set-up was developed which enables identical bonding pressure also for slightly different heights of sample assemblies. Both sample approaches were compared for various application examples: ranking of different classes of adhesives, validated statistics of adhesive-bonded joints using one particular adhesive and effects of cleaning and plasma-treatment on the adhesive strength of coatings. The investigated systems included glass, metals and polymer substrates, different adhesives, metallic and dielectric coatings on polymers.
Multiple-sample approach: reliable ranking and validated statistics of bonding and adhesive strength
(2014)
Bonding strength of adhesive-bonded joints and adhesive strength of coatings have been exclusively determined in
single-sample tests using a tensile testing machine. Necessarily, the single-sample approach was also applied to the
corresponding bonding procedure. As a consequence, reliability and reproducibility of results were restricted. By
applying the newly introduced centrifuge technology, the multiple-sample approach was realized for simultaneous
tensile testing of up to eight samples under identical conditions without any disturbing shear-force effects. In order to
introduce the multiple-sample approach into the bonding procedure, a hydraulic bonding set-up was developed which
enables identical bonding pressure also for slightly different heights of sample assemblies. Both sample approaches
were compared for various application examples: ranking of different classes of adhesives, validated statistics of
adhesive-bonded joints using one particular adhesive and effects of cleaning and plasma-treatment on the adhesive
strength of coatings. The investigated systems included glass, metals and polymer substrates, different adhesives,
metallic and dielectric coatings on polymers.
Response of linear, branched or crosslinked polyethylene structures on the attack of oxygen plasma
(2014)
Linear, branched and crosslinked polyethylenes (PE) were exposed to the low-pressure oxygen plasma for 2–120 s. In the following the samples were washed with solvents to remove low-molecular weight oxidized material and to excavate the subjacent polymer structure for microscopic characterization. X-ray photoelectron spectroscopy (XPS) measurements provided information about changes in elemental composition and chemical structure of PE after plasma exposure and washing. The calculation of the concentration of tertiary C atoms using XPS data was a measure of branches and crosslinking in the polymer before and after exposure to oxygen plasma. Linear PE was most sensitive towards oxygen plasma and showed the highest concentration in tertiary C atoms after plasma exposure. On the other hand branched PE types, which possess originally more tertiary carbon atoms, have lost two-third of them after 2 s oxygen plasma exposure. Branched PE show also topological changes at their surface as detected by atomic force microscopy. Differential scanning calorimetry measurements confirmed strong changes in crystallinity and molecular orientation of linear PE already after 120 s exposure to the oxygen plasma interpreted as amorphization. These effects should be interpreted as result of crosslinking caused by the recombination of dangling bond sites.
Aerosol-based DBD - A simple way to provide polymers with functional groups for adhesion promotion
(2013)
Polypropylene (PP) foils were exposed to the plasma of an atmospheric dielectric barrier discharge (DBD) in presence of different aerosols. Pure liquids such as water, ethanol, and 1/1 mixtures ethanol/water (v/v) or aqueous Solutions of acetic acid, poly(acrylic acid), poly(vinyl alcohol), and ethylene glycolpoly(vinyl alcohol) copolymer were introduced into the DBD. Surface composition, the number of functional groups per 100 C atoms, and water contact angles were correlated with the energy density of DBD plasma exposure.
The wettability of the treated surfaces was investigated for different storage times. Peel strength of the thermally deposited aluminum layer to modified polymer surfaces revealed that the aerosol introduction strongly improves the adhesion between aluminum and polypropylene.
A new concept for molecular interface design in metal-polymer systems is presented. The main features of this concept are the replacement of weak physical interactions by strong covalent bonds, the flexibilization of the interface for compensating different thermal expansions of materials by using long-chain flexible and covalently bonded spacers between the metal and the polymer as well as its design as a moisture-repellent structure for hindering diffusion of water molecules into the interface and hydrolysis of chemical bonds.
For this purpose, the main task was to develop plasmachemical and chemical techniques for equipping polymer surfaces with monotype functional groups of adjustable concentration. The establishing of monotype functional groups allows grafting the functional groups by spacer molecules by applying usual wet-chemical reactions. Four processes were favoured for production of monotype functional groups by highly selective reactions: the plasma bromination, the plasma deposition of plasma polymers, the post-plasma chemical reduction of O-functionalities to OH-groups, and the chemical replacement of bromine groups by NH2-groups. The grafting of flexible organic molecules as spacers between the metal layer and polymer improved the peel strength of the metal. To obtain maximal peel strength of aluminium coatings to polypropylene films and occurrence of cohesive failure in the polypropylene substrate, about 27 OH groups per 100 C-atoms or 6 COOH groups per 100 C-atoms were needed. Introducing C6-11-aliphatic spacers 1 OH or COOH group per 100 C-atoms contributed about 60% of the maximal peel strength of the Al-PP system, i.e. 2 or 3 spacer molecules per 100 C-atoms were sufficient for maximal peel strength.