Hager, K. (2020). High-temperature materials for hot lithography [Diploma Thesis, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2020.75540
In Additive Manufacturing Technology, focusing on Stereolithography (SLA), the material range is mainly limited to materials such as mono- and multifunctional methacrylates, having a high light sensitivity, hence outstanding photoreactivity. Besides this fact, they offer high-temperature resistance as respective materials can reach a glass transition temperature (Tg) of higher than 150 C. However, applications in the fields of aerospace, tooling and automotive require materials stable up to temperatures > 200 C, thus (meth)acrylates are not suitable anymore. In order to overcome these difficulties and deliver high thermal stability, different thermosets consistent of an aromatic and/or heterocyclic ring structure are used. Commonly used high-temperature thermoset classes are polyimides, cyanate esters and bismaleimides, as they display glass transition temperatures far beyond 200 C. The main limitation for applying such materials to traditional SLA technology is their processing behaviour, as these materials tend to have high melting temperatures, hence such raw materials require elevated temperatures to achieve suitable processing viscosity. Hence, a material is needed that displays high reactivity as photopolymerizable resin (i.e. processable via SLA technology) and excellent (thermo)mechanical properties when cured (i.e. Youngs modulus > 4 GPa with combined heat deflection temperature > 300 C). As these exceptional material classes exhibit increased viscosities and high melting temperatures, the new SLA technology Hot Lithography serves as a promising platform for such an undertaking as it enables the processing of highly-viscous resins (up to 20 Pa s) at elevated temperatures (as high as 120 C). The aim of this work was to find new high-temperature resistant materials that can be processed via the Hot Lithography technology, focusing on excellent (thermo)mechanical properties of the final 3D-part. Various materials (e.g. aliphatic or aromatic bismaleimides) are tested, which are known for their high-temperature resistance. Further process-related improvements to the resin formulation are conducted to attain a new material for 3D-structuring with exceptional material properties. Materials are examined towards their reactivity upon light activation via photoreactor studies, photo differential scanning calorimetry (photo-DSC) and real-time near-infrared photorheology (RT-NIR-photorheology). Thermal stability and (thermo)mechanical properties of the final 3D-parts were analysed by performing simultaneous thermal analysis (STA), dynamic mechanical analysis (DMA), heat deflection temperature measurements (HDT) and tensile tests.
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