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Eingeladener Vortrag
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Grundsätzliches zur Frage der "UV-Beständigkeit" bzw. der "Lichtbeständigkeit" polymerer Werkstoffe
(2013)
Photo-oxidative ageing of polymers under defined exposure (UV, humidity) was investigated from two points of view: a) the ageing process Starts from the topmost surface and penetrates slowly towards bulk and b) radicals play an important role among ageing. Therefore, X-ray photoelectron (XPS) and electron spin resonance (ESR) spectroscopy were applied to show both, the progressive incorporation of oxygen during in-situ UV irradiation (XPS) and the formation of radicals (ESR) in the polymer. The intensity of ESR Signals served as measure for radical concentration. The Validation of this correlation was checked for polyethylenes (PE) equipped with different concentrations of Hostavin N30 UV stabilizer.
The outdoor weathering of polymers is mainly due to UV radiation, water, Pollution, humidity, temperature etc. To understand the effects of weathering, simulated tests need to be undertaken in lab scale. Such tests must be quite efficient and similar to the effects of natural weathering. This paper mainly focuses on the effects of UV radiation along with humidity on LDPE.
To determine a polymer’s spectral response of photo degradation, two main methods have been established. The so-called “filter technique” uses polychromatic radiation, which is filtered by a set of various sharp cut-on filters. This procedure provides separated reading points of an activation spectrum of the investigated material. With the "spectrographic technique” a specimen is irradiated with pectrally dispersed radiation, locally quasi monochromatically, and the local property changes.are related to the radiant exposure at the respective wavelengths. A third method, the irradiation of specimens with quasi monochromatic radiation behind interference filters, is rarely used.
These methods have advantages and disadvantages and therefore, there are limits for conclusions from these spectral data. Concerning the quasi monochromatic Irradiation of specimens the development of new UV radiation sources looks promising.
With the rapid development of inexpensive UV Light-emitting diodes (LEDs), new : sources of nearly monochromatic radiation in the UV ränge are available, even down to about 250 nm. To determine the spectral response, the use of such UV-LEDs in TO-18 ) and TO-39 metal cases combines the advantages of low cost and energy efficiency, stable spectral irradiance, temperature.and humidity control, larger specimen areas, and an easy and stable analysis.
Combinations of different UV LEDs are also discussed as replacement for xenon arc ' radiation sources, see patent application EP01528388A1.
Artificial weathering of LDPE under dry conditions (<10% of relative humidity) by UV radiation was monitored
by dielectric spectroscopy. For an apolar polymer like LDPE, dielectric study yields information
about the change in structure and dynamics of the system. The fluctuations of the polar groups which
are generated as a result of photooxidation such as carbonyl groups can be monitored as characteristic
relaxation processes like the αc or β mode. The dielectric loss characterized by typical dielectric parameters
like the activation energy and dielectric strength are strongly dependent upon changes induced
by photooxidation in the amorphous and crystalline domains. This paper discusses in detail how the foil
reached its mechanical disintegration. It was found that initial stages of weathering induce chain scission
and/or crosslinking in the amorphous domains of the polymer and the motional processes in the crystalline
regions are not affected. After this induction period, an increase in the degree of crystallization was
observed by employing differential calorimetry. The increasing crystallinity is considered as one of the
reasons for embrittlement which finally leads to the complete loss of the structural integrity.
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.
Eine Verkürzung der Dauer von Bewitterungsversuchen kann durch eine empfindlichere Detektion der Bewitterungseffekte erreicht werden als sie konventionell benützt wird. Eine solche besonders empfindliche Detektionsmethode stellt die Chemilumineszenz dar. Die Verkürzung durch empfindliche Detektion muss jedoch gleichzeitig der Bedingung genügen, ausreichend gute Rangkorrelation zu Bewitterungseffekten unter Anwendungsbedingungen zu zeigen. In diesem Spannungsfeld sollen die Möglichkeiten der Chemiluminiszenz-Detektion beleuchtet werden.
Accelerated weathering test for plastics and coatings that are exposed to acidic precipitation
(2009)
Due to the occurrence of acidic precipitation, and due to the fact that polymers and stabilizers may be sensititve to acids, it is important to have a laboratory test that combines weathering exposure with acidic exposure. The loss of stabilization caused by an acid damage of stabilizing system gets evident only over the weathering progress. For solid samples, for wich mostly the surface properties are relevant, the "Acid Dew and Fog Test" (ADF test) was established. For woven or tufted materials, the "Acid Rain Test" (AR test) was developed. The proposed weathering tests aim at plastics as well as at coatings.