Due to the easy accessibility and availability, the interest in solar energy as renewable energy source has grown rapidly within recent years. Solar energy can be stored either indirectly as electric energy using photovoltaics or directly as thermal energy in solar thermal systems. To be able to compete with energy prices from non-renewable energy sources, commercially available solar systems have to be optimized in terms of the costs. Within the SolPol-4/5 project that provides the framework of this thesis, inexpensive polymeric materials used for the absorbers and the hot water heat stores were developed and evaluated according to their mechanical stability and degradation behavior. To guarantee a required durability of at least 20 years, various polymer stabilizers and additives are usually employed. In the best case, stabilizers with different mechanisms of actions complement each other, thereby contributing to an enhanced material stability. ^However, there are certain stabilizer combinations leading to unpredictable antagonistic interactions, making it necessary to investigate interaction reactions before preparing real polymer materials. To obtain such results within an acceptable time, accelerated aging experiments of polymer samples containing the stabilizers of interest are usually performed. To save time and reduce costs even further, within this thesis the polymer-mimicking solvent squalane was employed and tested in terms of applicability and validity. ^The major advantage of squalane arises from the fact that it is liquid at ambient conditions allowing to simply dissolve the stabilizers in the matrix without requiring a complex and time-consuming polymer extrusion process.
The first publication presented in this thesis demonstrates that squalane may be perfectly suited for rating a large number of polymer stabilizers and combinations thereof using an approach based on high performance liquid chromatography (HPLC) and fluorescence detection. Similar to polypropylene, squalane reveals aging-induced photoluminescence emissions, which correlate with the extent of the matrix degradation and may therefore be used to discover synergistic and antagonistic stabilizer interaction phenomena. ^Investigation of the photoluminescent squalane signal applying high-resolution Orbitrap mass spectrometry (MS) revealed a formation of unsaturated carbonyl squalane compounds with different chain lengths.
The extraction of the stabilizer-containing squalane samples with organic solvents additionally allows the investigation of the various reaction and degradation products of stabilizers, which is presented in the second research paper of this thesis. The study of the degradation products of hindered amine light stabilizers (HALS) and phenolic antioxidants using HPLC coupled to UV and high-resolution quadrupole time of flight MS detection enabled the elucidation of the observed synergism under UV-light. It was shown that the phenolic antioxidant Irganox 1330 is oxidized to a conjugated quinoid derivative, thereby developing UV-light-absorbing abilities. The oxidized antioxidant acts like a UV-absorber preventing or slowing down the degradation of the squalane matrix. ^However, without the protection of the HALS, the phenol may not develop its activity since it is degraded by the UV-light, so that this interaction comprises a synergistic effect. The stabilizer formulations applied in squalane were additionally tested in real polymer samples. The polymer films were analyzed with respect to stabilizer concentrations and changes of molecular weight during accelerated aging experiments.
Assuming that stabilizers are distributed homogeneously within polymer materials, a monitoring of the concentrations during aging may provide useful information about the degradation behavior. It can however be expected that material failures like crack formation are caused by stabilizer inhomogeneities. The third paper of this thesis is dedicated to the imaging of polymer additives within a polypropylene matrix applying confocal fluorescence microscopy with a fluorescent whitening agent as stabilizer model compound. ^The superior method sensitivity enabled the imaging of samples containing less than 0.5 mass percent (wt%) additive without sacrificing time and lateral resolution as it is the case for less sensitive imaging techniques like IR or Raman microscopy.