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Paper des Monats
- ja (3)
Melamine and melamine resins are widely used as fire retardants for polymer materials used in pharmaceutical, plastic, textile, rubber, and construction industry. Melamine-based flame retardants act by blowing off intumescent layers, char formation, and emission of quenching ammonia gas and diluent molecular nitrogen. Special advantages are: low cost, low smoke density and toxicity, low corrosive activity, safe handling, and environmental friendliness. Methylated poly(melamine-co-formaldehyde) (mPMF) was used as thick (≥40 µm) fire-retardant coating for plasma pretreated polymers. A combined low-pressure plasma pretreatment consisting of oxygen plasma exposure followed by deposition of thin poly(allylamine) (ppAAm) and poly(allyl alcohol) (ppAAl) plasma polymers as adhesion promoters have improved the adhesion of thick mPMF coatings strongly. Chemical structure and composition of deposited polymer films were characterized by infrared-attenuated total reflectance and X-ray photoelectron spectroscopy (XPS). After peeling, the peeled layer surfaces were also investigated for identification of the locus of failure and their topography using optical microscopy and XPS. Often the adhesion promotion was so efficient that the peeling of coating was not possible. Thermal properties of plasma polymers and dip-coating films were analyzed by thermogravimetric analysis. Significant improvement of fire-retardant properties of coated polymers was confirmed by flame tests.
The adhesion of thick poly(allylamine)-polyphosphate layers (1 µm) deposited by the wet-chemical layer-by-layer technique (LbL) onto polyethylene or polystyrene (each 100 µm) was very low. To promote the adhesion of these LbL layers the polyolefin substrates were oxidized at the surface by short exposure to the oxygen plasma (2 or 5 s) and subsequently coated with an interlayer of plasma-deposited poly(allylamine) or poly(allyl alcohol) (100 nm). The plasma polymer interlayers have improved strongly the adhesion between polyolefin substrates and polyphosphate coatings. Such phosphate coatings are interesting for life sciences (nucleotide formation) but also for fire retardancy in combination with N-rich compounds such as melamine.
The intention was to prefer chemical and hydrogen bonds for adhesion promoting because of their high binding energy. Therefore, the introduced oxygen-containing groups at the polyolefin surface could interact with the OH or NH2 groups of the adhesion-promoting plasma polymer interlayer. These groups were also able to interact strongly with the poly(allylamine)-polyphosphate topcoating.
The coated polyolefins were investigated using Fourier Transform Infrared Spectroscopy in Attenuated Total Reflectance mode (FTIR-ATR), X-ray Photoelectron Spectroscopy (XPS), Thermo-Gravimetric Analyses (TGA) and Atomic Force Spectroscopy (AFM) and 90° peel test.
Analyse häufig verwendeter europäischer Papiere. Im Fokus: Fasermaterial, Füllstoffe und Leimung
(2007)
Restauratoren können verschiedene Papierarten mit dem bloßen Auge aufgrund von Merkmalen in der Textur und der Oberflächenbeschaffenheit, seinem Aussehen im Durchlicht sowie an Griff und Klang grob unterscheiden. Dabei ist sich der Restaurator stets der Tatsache bewusst, dass es sich lediglich um eine erste grobe Einschätzung handeln kann. Erst bei mikroskopischen Analysen und physikalischen Experimenten treten Charakteristika zu Tage, die eine weitere Annäherung an die Zusammensetzung des Papiers und damit seiner Provenienz erlauben. Gegenstand dieser Arbeit war es, ein europäisches Hadernpapier, ein holzhaltiges Zeichenpapier und ein Whatman®-Filterpapier mit zwei mikroskopischen Methoden und einer Verbrennungstechnik zu untersuchen. Das Fasermaterial, die Füllstoffe, die Leimung und eventuelle Verunreinigungen wurden detailliert evaluiert. Neben konventioneller Lichtmikroskopie kam die weniger bekannte Umweltrasterelektronenmikroskopie (ESEM = Environmental Scanning Electron Microscopy) zum Einsatz, die neben der bildlichen Darstellung der Probenoberflächen zusätzlich eine Elementanalyse derselben gestattet. Letzteres erfordert allerdings das Vorhandensein eines zusätzlichen Röntgenspektrometers. Darüber hinaus wurden die verschiedenen Papiere trocken verascht. Es wurde eine oxidative Zerstörung der organischen Stoffe gewährleistet bis nur noch nichtbrennbare mineralische Asche zu verzeichnen war. Die Komplementarität der eingesetzten Methoden wird demonstriert und ein Einblick in die Zusammensetzung und Struktur der Papiere ermöglicht. Die Analysemethoden sind durch ihren partiell zerstörenden Charakter bzw. Limitierungen hinsichtlich der Objektgröße nur bedingt für die Untersuchung am Original geeignet, sehr wohl aber dafür, das Probenmaterial für Versuchsreihen genau zu definieren und zu standardisieren.
Laser-induced periodic surface structures (LIPSS, ripples) are a universal phenomenon that can be observed on almost any material after the irradiation by linearly polarized laser beams, particularly when using ultrashort laser pulses with durations in the picosecond to femtosecond range. During the past few years significantly increasing research activities have been reported in the field of LIPSS, since their generation in a single-step process provides a simple way of nanostructuring and surface functionalization towards the control of optical, mechanical or chemical properties. In this contribution current applications of LIPSS are reviewed, including the colorization of technical surfaces, the control of surface wetting, the tailoring of surface colonization by bacterial biofilms, and the improvement of the tribological performance of nanostructured metal surfaces.
The formation of laser-induced periodic surface structures upon irradiation of titanium, silicon, and fused silica with multiple irradiation sequences consisting of parallel polarized Ti:sapphire femtosecond laser pulse pairs (pulse duration 50–150 fs, central wavelength ~800 nm) is studied experimentally. The temporal delay between the individual near-equal energy fs-laser pulses was varied between 0 and 5 ps with a temporal resolution of better than 0.2 ps. The surface morphology of the irradiated surface areas is characterized by means of scanning electron microscopy (SEM). In all materials a decrease of the rippled surface area is observed for increasing delays. The characteristic delay decay scale is quantified and related to material dependent excitation and energy relaxation processes.
Wer mit Laserstrahlung zu tun hat, sollte vor allem auf den Schutz seiner Augen achten, denn die Netzhaut ist eines der empfindlichsten Gewebe des menschlichen Körpers. Auch die Hornhaut des Auges kann durch Laserstrahlung gefährdet sein. Sie hat eine Dicke von nur etwa 0,5 mm. Der Schutz der Augen vor Laserstrahlung ist durch verschiedene Maßnahmen möglich.
In this study, femtosecond laser-induced sub-micrometer structures are generated to modify polyethylene (PE) surface topographies. These surfaces were subjected to bacterial colonization studies with Escherichia coli and Staphylococcus aureus as test strains. The results reveal that the nanostructures do not influence S. aureus coverage, while the adhesion of E. coli is reduced.
Optical fibers made of fused silica are a common method of transmitting high laser pulse energies. Failure of those fibers is a significant risk. The determination of laser-induced damage thresholds (LIDT) on fiber end facets according to ISO 21254 standard is needed. In the past, single pulse nanosecond laser experiments showed an improvement of LIDT with increasing fiber core diameter for 1064 nm wavelength and a constant beam diameter of 50 µm.
This paper pays particular attention to the influence of the laser beam diameter on damage resistance. All-silica fiber types (LEONI) with different core diameters (100–600 µm) were investigated using beam diameters in a range from 30 µm to 100 µm. For comparison experiments on fused silica preform material (Heraeus F300) were performed. On one hand, surface LIDT of fused silica preform material decreases significantly with increasing beam size. A model considering a random distribution of point defects explains the experimental data qualitatively. On the other hand, LIDT of fiber end facets stays constant. White light microscopy results suggest that the point defect density on fiber end facets is lower compared to the preform surface due to an excellent surface polish quality.
Zusammenfassung
The surface chemical modification of polyurethane (PU) films was performed by an UV laser-induced chemical reaction in a polysaccharide solution. This process may be applicable as hydrophilic packaging of implantable medical devices and in vivo sensors. When a PU film in contact with an aqueous alginic acid (AAC) solution was irradiated with a XeCl laser, the PU film turned hydrophilic. Contact angles of water on the film were reduced from 110° to 60°. Since light absorption of the AAC solution at 308 nm was negligibly small, reactive sites were generated solely on the PU surface. There, AAC could be immobilized by chemical bonds thus allowing for a nanometer-scaled grafting of this biomolecule. The mechanism was investigated by surface analyses with Fourier-transform infrared spectroscopy (FT-IR), dye staining, ultraviolet-visible (UV-VIS) spectroscopy, and scanning electron microscopy (SEM) techniques. A one-photon photochemical process could beidentified.
For many applications, optical multimode fibers are used for the transmission of powerful laser radiation. High light throughput and damage resistance are desirable. Laser-induced breakdown at the end faces of fibers can limit their performance. Therefore, the determination of laser-induced damage thresholds (LIDT) at the surface of fibers is essential.
Nanosecond (1064 nm and 532 nm wavelength) single-shot LIDT were measured according to the relevant standard on SiO2 glass preforms (Suprasil F300) as basic materials of the corresponding fibers. For 10 kinds of fused silica fibers (FiberTech) with core diameters between 180 µm and 600 µm, an illumination approach utilizing a stepwise increase of the laser fluence on a single spot was used. For both wavelengths, the LIDT values (0% damage probability) obtained by means of the two methods were compared. The influence of surface preparation (polishing) on damage resistance was investigated. For equal surface finishing, a correlation between drawing speed of the fibers and their surface LIDT values was found. In addition to the surface measurements, bulk LIDT were determined for the preform material.
Censorship of parts of written text was and is a common practice in totalitarian regimes. It is used to destroy information not approved by the political power. Recovering the censored text is of interest for historical studies of the text. This paper raises the question, whether a censored postcard from 1942 can be made legible by applying multispectral imaging in combination with laser cleaning. In the fields of art conservation (e.g. color measurements), investigation (e.g. Analysis of underdrawings in paintings), and historical document analysis, multispectral imaging techniques have been applied successfully to give visibility to information hidden to the human eye.
The basic principle of laser cleaning is to transfer laser pulse energy to a contamination layer by an absorption process that leads to heating and evaporation of the layer. Partial laser cleaning of postcards is possible; dirt on the surface can be removed and the obscured pictures and writings made visible again. We applied both techniques to the postcard. The text could not be restored since the original ink seems to have suffered severe chemical damage.
Thin plasma polymer films were deposited using the pulsed plasma mode. These plasma polymers should possess a more regular structure than those produced by the conventional continuous-wave (cw) mode, because of lower monomer fragmentation caused by the plasma pulses and the chemical chain propagation during the plasmaless (free!) periods. The thermoluminescence method was applied to functional groups carrying plasma polymer layers which are used in medical technology. Examples are formation of biocompatible, biosensoric and bioactive coatings or in metal polymer composites such as adhesion-promoting interlayers.
In addition to the use of the conventional X-ray Photoelectron Spectroscopy for thin film characterization, the new method of thermoluminescence was applied to characterize undesired defects and structural specifics produced in the polymer films by pp or cw plasma mode. The main areas of focus were oxygen-containing groups produced by post-plasma oxygen introduction via auto-oxidation, oxidation of implemented unsaturations and trapped radical sites known as typical irregular structures in plasma polymers.