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Analysis of microplastic particles (MP) in environmental samples needs sophisticated techniques and is time intensive due to sample preparation and detection. An alternative to the most common (micro ) spectroscopic techniques, FTIR or Raman spectroscopy, are the thermoanalytical methods, where specific decomposition products can be analyzed as marker compounds for different kind of plastics types and mass contents. Thermal extraction desorption gas chromatography mass spectrometry (TED-GC-MS) allows the fast identification and quantification of MP in environmental samples without sample preparation. Whereas up to now only the analysis of thermoplastic polymers was realized, this is the first time that even the analysis of tire wear (TW) content in environmental samples is possible. Various marker compounds for TW were identified. They include characteristic decomposition products of elastomers, antioxidants and vulcanization agents. Advantages and drawbacks of these marker substances were evaluated. Environmental samples from street run off were exemplarily investigated and presented.
Thermoset materials characterization is often limited to solid state analytical techniques such as IR, NMR, DSC, TGA and mechanical testing. Alternatively, their off-gassing behavior can also be evaluated using GC based techniques such as TD-GC-MS, allowing this method to be applied to thermoset materials analyses such as identification, aging characterization, and formulation optimization. As an overview, common thermoset materials were evaluated by analyzing their gaseous degradation products via TGA-based pyrolysis and subsequent TD-GC-MS for the identification of representative volatile signatures. It is thereby possible to distinguish different classes of phenolic materials or cured epoxy resins, as well as their amine or anhydride curatives. Additionally, this method enabled quantification of a volatile fragment (bisphenol A, BPA) which is associated with oxidation of epoxy/amine thermoset materials. The amount of evolved BPA increased linearly with aging time and this trend exhibits linear Arrhenius behavior over the temperature range (80–125 °C) studied, in agreement with oxidation sensitivies based on oxygen consumption data. Further, TD-GC-MS was used to explore how off-gassing of residual anhydride curative from an epoxy/anhydride material depends on formulation stoichiometry. Even in formulations that theoretically contained enough epoxy to consume all anhydride (1:1 stoichiometry), an imperfect final cure state resulted in residual anhydride which could evolve from the material. For such materials, a slightly epoxy-rich formulation is required to ensure that the material contains no residual unreacted anhydride. Analysis of volatiles generated by thermal exposure is an attractive characterization approach enabling compositional analysis as well as complementary diagnostics for materials degradation.
Due to the high molecular weights the analysis of polymers is difficult to carry out. Their dissolution is usually limited and the vapor pressure is very low. However, the degradation of a polymer leads usually to smaller compounds which then enable volatile analysis to aid in the characterization of the degradation or decomposition mechanism. The type and the amount of these smaller degradation products is characteristic for the momentary stage of the degradation process.
With thermal desorption coupled to gas chromatography with mass spectrometry (TDS-GC-MS) the polymers can be heated from ambient temperatures to approximately 300 °C. During the heating process small volatile products with a molecular mass up to 350 - 400 m/z were first cryogenic trapping, reheated under controlled conditions and finally separated through a chromatographic column and identified with mass spectrometry. For instance clear differences in the released volatile species of Polypropylene (PP) can be observed depending on the progress of accelerated aging.
In combination with a Thermogravimetric analyzer (TGA) the samples can be heated up to 1000 °C under inert as well oxidative atmosphere. This leads to a complete thermal-pyrolytic or thermo-oxidative degradation of the whole material and a release of degradation fragments over a range of Mw. These fragments enable conclusions for the underlying degradation or decomposition pathways. For coupling to GC-MS a solid-phase adsorption agent can be coupled to the exhaust of the TGA. This adsorption agent is usually a PDMS species that is capable to adsorb compounds with a wide range of polarities. After the thermal extraction the solid-phase adsorption agent will be analyzed by TDS-GC-MS. Therefore, it is possible to determine the species and amounts of the released complex thermal or thermo-oxidative degradation products in detail. This approach offers overall advantageous over existing volatile analytical methods.
Thus, it was possible to determine new thermal and thermo-oxidative degradation mechanism pathways of a well-known material like Polyamide 66 (PA 66). Further example is the unique analysis of small polymer particles, which can be identified and quantified in complex environmental samples. This is our current focus of R&D activities within the framework of the analysis of microplastics in the environment. In the present work we will present a new, automatized set up of this method.
Using thermogravimetric analysis (TGA) with a solid-phase adsorber for thermal extraction, followed by subsequently analysing the adsorber with thermo-desorption gas chromatography mass spectrometry (TDS-GCMS) enables measurement of polymer degradation under oxidizing atmosphere, and the identification of certain complex hydrocarbon degradation products by chromatographic separation and defined mass patterns. This technique, thermal-extraction desorption gas chromatography mass spectrometry (TED-GCMS) was used to investigate the thermo-oxidative degradation of PA 66 and PA 66 doped with 2 wt% of metal oxide particles. In TGA pure PA 66 formed more residue under an oxidizing atmosphere than an inert one. In contrast to the measurements under inert atmosphere, several condensed aromatic species containing nitrogen could be identified in thermo-oxidative measurements. These degradation products were formed through condensation reactions of primary amides originating from imide hydrolysis. The formation of such highly condensed species also causes higher char formation. Four metal oxides have shown an impact on the thermo-oxidative degradation of PA 66: Fe2O3 on η-Al2O3 < pure Fe2O3 = Fe2O3 on γ-Al2O3 < pure ZnO. For ZnO even a char-stabilizing effect could be observed. A catalytic effect of these metal oxides causes more condensed cyclopentanone and pyridine derivates. Thus, more water is formed and released, resulting in increased hydrolysis of the imides and degradation at lower temperatures.
The impact of metal oxide particles on the thermal and thermo-oxidative degradation of polyamide 66
(2014)