The thesis contains a background and reflections section, an introduction, and three appended articles. The first section is reserved for some of the background and basics on polymers and polymer composites and a discussion on their effect on our everyday lives. The introduction gives a brief recap of the project. The articles contain the research that was performed on the thermal oxidation of thermosetting polyimides and their composites during the project. The first article covered the thermal oxidative degradation of satin weave and thin-ply composites made by resin transfer molding with carbon fibers and thermosetting polyimide. The degradation was studied by weight loss measurements and X-ray computed microtomography. The weight loss measurements showed that the initial desorption stage during ageing followed Fickian behavior and the proposed model. It was also observed that the satin weave composites formed crack clusters that grew into a network of cracks, voids and delaminations throughout the specimens as the ageing time progressed, while the thin-ply composites only formed delaminations at the free edges. The second manuscript studied the behavior of the neat polyimide resin when aged for up to 1500 hours in ambient air, and compared it with a newly developed polyimide formulation, with slightly altered chemical composition. The reduced amount of internal crosslinkers in the newer formulation was expected to enhance the fracture toughness of the material. Three-point bending, differential scanning calorimetry, dilatometry, weight loss, light optical microscopy and nanoindentation experiments were performed and highlighted the differences in the thermal and mechanical properties of the two formulations. A slight increase in the fracture toughness was observed, while the glass transition of the new formulation had decreased. The third manuscript was aimed at continuing the discussion from the second article on the differences between the two thermosetting polyimides. Thermogravimetric scans showed that the polyimide formulations behaved very similarly under thermal oxidative tests. The initial analysis gave indications that the model could capture well the degradation at high temperatures, but is not adequate in predicting long-term degradation at temperatures around 288–400 °C.

The thermal-oxidative degradation of a crosslinked hydroxy-terminated polybutadiene (HTPB)/cycloaliphatic diisocyanate (H12MDI) based polymer, which is commonly used as a polymeric binder in propellants, is investigated at temperatures from 95°C to 125°C with the aim of estimating the lifetime of the material in storage conditions (20°C) using the Arrhenius approach. Furthermore, the effect of antioxidants and to a lesser extent plasticizer on the degradation process was also studied. Diffusion-limited oxidation (DLO) was theoretically modelled and DLO conditions were estimated by gathering oxygen permeability and consumption data from similar studies. It was concluded that DLO-effects might be present at the highest experiment temperature (125°C) depending on the actual properties of the material investigated. The mechanical degradation was monitored by conducting tensile tests in a DMA apparatus and photographs using a microscope was taken to examine potential DLO effects. The degradation process of the stabilized polymer (with antioxidant) did not showcase Arrhenius behaviour, which was confirmed by the failure to construct a satisfactory mastercurve. This was most likely due to loss of antioxidants, resulting in autocatalytic oxidation(acceleration of the oxidation process). However, the induction period of the stabilized polymer showcased Arrhenius behaviour in the temperature region 95-125°C with an ~E_a = 90 kJ/mol. If the activation energy E_a is assumed to remain constant, the lifetime at ambient temperature (20°C) is predicted to be approximately 176 Years for a 2mm thick sample. However, this is probably an overestimation since curvature in the Arrhenius plot has been observed for many rubber materials in the lower temperature region. Assuming the E_a drops from ~90 kJ/mol to~71 kJ/mol, a more conservative lifetime prediction of 58 Years was estimated.

Das Infrarotschweißen ist ein industriell etabliertes Verfahren zur Herstellung von Bauteilen in unterschiedlichsten Anwendungsbereichen. Die Prozesseinrichtung ist jedoch mit einem hohen Kosten- und Zeitaufwand verbunden, da komplexe Strahler-Werkstoff-Wechselwirkungen das Aufschmelzverhalten des Kunststoffbauteils bestimmen. In vielen industriellen Infrarotschweißprozessen ist daher ein Rauchen der infraroterwärmten Bauteilbereiche zu beobachten. Eine Erforschung des Zusammenhangs zwischen Rauchbildung, Kunststofftemperatur, thermisch-oxidativer Belastung des Kunststoffs und den resultierenden mechanischen Schweißnahteigenschaften steht bislang aus. Weiterhin sind in Infrarotschweißprozessen in der industriellen Fertigung oftmals hohe Umstellzeiten und schwankende Fügeteiltemperaturen festzustellen. In der vorliegenden Arbeit werden erstmals die mechanischen Eigenschaften von Infrarotschweißverbindungen mit der Rauchbildung und thermisch-oxidativen Kunststoffbelastung korreliert und zwei Ansätze zur schonenden Erwärmung untersucht. Die Ergebnisse weisen nach, dass eine thermisch-oxidative Kunststoffschädigung zu einer Verschlechterung der mechanischen Schweißnahteigenschaften führt und bei der Auslegung industrieller Prozesse in Betracht gezogen werden muss. Das Schweißen in Argonatmosphäre und mit aktiver Strahlerleistungsregelung verhindern die Kunststoffzersetzung und führen in der Regel zu besseren mechanischen Schweißnahteigenschaften. Weiterhin wird deutlich, dass die Fügeteiltemperatur einen vernachlässigbaren Einfluss auf die Schweißnahteigenschaften hat, wohingegen Umstellzeiten ≥ 5 s eine erhebliche Reduzierung ebendieser zur Folge haben. Eine weitere Herausforderung ist das Infrarotschweißen (endlos-)faserverstärkter Kunststoffe. Die derzeit übliche Stumpfanordnung der Fügeteile führt zu einer Faserumlenkung in der Fügenaht und hat zur Folge, dass die Faserverstärkung nicht über die Fügeebene hinweg genutzt werden kann. Im Rahmen der Arbeit wird aufgrund dessen das überlappende Infrarotschweißen von Organoblechen untersucht. Um Bauteile aus Organoblechen mit erhöhter Komplexität, Größe und Steifigkeit herstellen zu können, werden zudem zwei industriell nutzbare Verfahrensvarianten auf Basis des Infrarotschweißens entwickelt. Unter Nutzung dieser, können Organoblechhohlkörper mit Überlappverbindungen gefertigt werden. Die Ergebnisse zeigen, dass sowohl in Plattenprobekörpern als auch in Hohlkörpern eine Nutzung der Faserverstärkung über die Fügeebene hinweg möglich ist.:1 Einleitung und Zielsetzung 2 Grundlagen und Stand der Technik 3 Experimentelles 4 Analyseverfahren 5 Ergebnisse zum Stumpfschweißen 6 Ergebnisse zum Überlappschweißen 7 Bewertung der Ergebnisse 8 Zusammenfassung und Ausblick / The infrared welding is a well-established process in the industrial production of parts in various applications. However, the complex emitter-material interaction, which influences the meltdown behaviour of the plastic parts, results in a high effort for the process setup. A smoking of the plastic parts is to observe in numerous industrial infrared welding processes. The correlations between the smoking of the plastic, its temperature and thermal-oxidative degradation as well as the mechanical properties of the resulting welds are unidentified yet. Furthermore, in industrial infrared welding processes often high changeover times and varying joining part temperatures are existent. Therefore, within the present work the connections between the mechanical joint properties of infrared welds and the thermal-oxidative degradation of plastics are elaborated for the first time and two approaches for the gentle infrared heating are investigated. The findings prove that the thermal-oxidative degradation of the plastic substantially decreases the mechanical weld properties and needs to be taken into account when setting up the industrial infrared welding process. The welding in argon atmosphere and the use of an active infrared emitter power control, which ensures the heating of the plastic below the degradation temperature, lead to better mechanical weld properties. In addition, the outcome of this work shows that the influence of the temperature of the joining part is negligible regarding the mechanical joint properties, whereas changeover times greater than or equal to 5 s lead to a dramatical decrease in the mechanical properties. Another challenge is the infrared welding of fibre reinforced plastics. The butt welding of fibre reinforced thermoplastics is common practice and prevents the use of fibres in the joint plane due to the fibre deflection in this area. As a result, the overlapping welding of organo sheets is investigated as well. In order to produce large and complex parts with high stiffness made of organo sheets, two process variants on the basis of the infrared welding technology are developed, which can be used at the industrial scale to manufacture hollow bodies. The overlapping welds of specimens and in hollow bodes made of organo sheets, enable the fibre utilisation across the joint plane.:1 Einleitung und Zielsetzung 2 Grundlagen und Stand der Technik 3 Experimentelles 4 Analyseverfahren 5 Ergebnisse zum Stumpfschweißen 6 Ergebnisse zum Überlappschweißen 7 Bewertung der Ergebnisse 8 Zusammenfassung und Ausblick