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Paper des Monats
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Chemiluminescence (CL) is a sensitive method of investigating the thermo-oxidative stability of polymers in the early stages of degradation. Therefore, CL measurements were applied to different automotive coatings to evaluate their degradation behavior during artificial weathering. In this work, CL measurements were carried out for different automotive coatings to follow their degradation behavior during artificial weathering. The CL emission depends on sample-specific parameters like stabilizer, matrix, or hardener; experimental parameters like the oxygen partial pressure; and exposure parameters like the duration of weathering. The potential of CL could be demonstrated by classifying materials into categories of good, medium, and low degradation performance at a much earlier stage of weathering exposure than in combination with the usual visual detection of weathering effects.
Thermally stimulated luminescence (TL) can be used to detect damage in the early stages of degradation after artificial weathering, and to analyze the efficiency of stabilizers used in automotive coatings. TL is particularly suitable for this task due to its inherent high sensitivity to structural, morphological, and chemical changes in macromolecular chains. In this work, TL measurements were carried out for different automotive coatings to follow their degradation progression during artificial weathering. The TL results demonstrate high sensitivity to parameters like stabilizers, matrices, hardeners, and weathering time, and provide an opportunity to monitor the early stages of damage in polymers, which therefore reduce the time required for further outdoor weathering tests.
The thermo-oxidative degradation of a polymeric optical cable is investigated by chemiluminescence, The results are reliable and reproducible. Two distinct processes are reported marked by a peak and a plateau behavior versus the time, respectively. Both processes are ruled by thermally activated processes. Beside the dependencies of temperature and time, the influence of absorbed water is discussed. Chemiluminescence is proposed as a promising candidate for a suitable testing method assessing the thermo-oxidative stability of plastic optical fibers and cables. it requires not more than a simple one-day testing procedure and has the advantage that it can be carried out even within the lo cv temperature ranges of the cables' intended use.
The excitation energy transfer (EET) of a bichromophoric cross-shaped molecule was investigated by stationary polarized fluorescence spectroscopy in the solid state. For this purpose 2,2[prime],7,7[prime]-tetrakis(biphenyl-4-yl)-9,9[prime]-spirobifluorene was embedded in a polymeric bisphenol-A-polycarbonate (PC) matrix. The dependence of the fluorescence on concentration and wavelength was determined. The role of the intermolecular and intramolecular EET is dealt with separately and discussed by means of the degree of polarization. The intermolecular excitation energy transfer is described in terms of a Förster transfer mechanism. The intramolecular transfer is prevented for the zero-point vibrational levels by the molecular cross-shaped structure, but is found for a wide range of wavelength, presumably based on vibrationally excited states.
Degradation effects of automotive coatings after weathering as detected by thermoluminescence
(2004)
Angesichts der spezifischen Bedeutung der Aushärtung von Phenolharzen für ihre Eigenschaften als Ingenieurwerkstoffe besteht großes Interesse an deren quantitativer Charakterisierung. Mittels DSC-Messungen unter hohem Druck (HP-DSC) kann die Vernetzungsreaktion der Phenolharzformmassen durch die Unterdrückung der Verdampfung niedermolekularer Substanzen ohne störende Überlagerungen verfolgt werden. Mittels verschiedener Vernetzungsgrade können die Einflussparameter der Aufheizrate und der Nachtemperung optimiert werden. Die Messergebnisse ermöglichen eine Modellierung der Reaktionskinetik mit dem Ziel einer verbesserten Prozessführung und Qualitätskontrolle in der industriellen Praxis.
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.
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.