2022-11-23, Schär, Matthias, Zweifel, Lucian, Arslan, Delal, Grieder, Stefan, Maurer, Christoph, Brauner, Christian

This paper describes the application of poly(ether-block-amide) polymers, so-called Pebax, in fused filament fabrication (FFF). Pebax® is a thermoplastic elastomer (TPE), a copolymer based on rigid polyamide and soft polyether blocks. By variation of the blocks, unique properties such as soft or rigid behaviour are tailored without additional additives and plasticisers. Pebax®Rnew® polyamide blocks are bio-based and made from castor beans that allow the design of sustainable applications. In this study, two types of Pebax were selected, processing parameters were characterised, filaments were extruded and applied to FFF printing, and the final mechanical characteristics were determined. Both types were suitable for FFF processing with improved process stability due to less shear thinning and good mechanical performance. The connection strength between the grades was also described in the design context for complex parts with tailored soft or hard regions. Combining the two materials in one design is a promising concept, and the adhesion strength is close to the strength in the Z-direction of the flexible Pebax®Rnew®35R53 grade.

2021-10-12, Zweifel, Lucian, Zhilyaev, Igor, Brauner, Christian, Rheme, Martin, Eckhard, Gregor, Bersier, Valentin, Glavaški, Slobodan, Pfeiffer, Ricardo

Creating connection points for sandwich-structured composites without losing technical performance is key to realising optimal lightweight structures. The patented LiteWWeight® technology presents cost-effective connections on sandwich panels in a fraction of a few seconds without predrilling. Ultrasonic equipment is used to insert a thermoplastic fastener into the substrate material and partially melt it into the porous internal structure. This creates a highly interlocked connection (connection strength is above 500 N) suitable for semi-structural applications. This study focused on the simulation and experimental validation of this process, mainly on the interaction between the pin and the substrate material during the joining process. The dynamic thermo-mechanical model showed reasonable agreement with experimental methods such as process data, high-speed camera monitoring or computed tomography and allowed the prediction of the connection quality by evaluation of the degree of interlock. The connection strength prediction by the developed model was validated within several various process setups, resulting in a prediction accuracy between 94–99% depending on the setup.

2022-02-08, Zweifel, Lucian, Ritter, Klaus, Brauner, Christian

The joining of composites mostly relies on traditional joining technologies, such as film or paste adhesives, or mechanical fasteners. This study focuses on the appealing approach of using standard thermoplastic welding processes to join thermosets. To achieve this, a thermoplastic coupling layer is created by curing with a thermoset composite part. This leads to a functional surface that can be utilized with thermoplastic welding methods. The thermoplastic coupling layer is integrated as a thin film, compatible with the thermoset resin in the sense that it can partially diffuse in a controlled way into the thermoset resin during the curing cycle. Recent studies showed the high affinity for the interphase formation of poly hydroxy ether (phenoxy) film as coupling layer, in combination with a fast-curing epoxy system that cures within 1 min at 140 °C. In this study, an investigation based on resistance and ultrasonic welding techniques with different testing conditions of single-lap shear samples (at room temperature, 60 °C, and 80 °C) was performed. The results showed strong mechanical strengths of 28.9 MPa (±0.7%) for resistance welding and 24.5 MPa (±0.1%) for ultrasonic welding, with only a minor reduction in mechanical properties up to the glass transition temperature of phenoxy (90 °C). The combination of a fast-curing composite material with an ultra-fast ultrasonic joining technology clearly demonstrates the high potential of this joining technique for industrial applications, such as automotive, sporting goods, or wind energy. The innovation allowing structural joining performance presents key advantages versus traditional methods: the thermoplastic film positioning in the mold can be automated and localized, joint formation requires only a fraction of a second, and the joining operation does not require surface preparation/cleaning or structure deterioration (drilling).

2020-12, Zweifel, Lucian, Brauner, Christian

This study focuses on investigating thermoplastic polymers and their interphase behaviour with the fast-curing epoxy system Araldite LY3585/Aradur 3475. The three compatible thermoplastic polymers poly(methyl methacrylate) (PMMA), amorphous polyamide (PA12TR90), and poly(hydroxy ether) (phenoxy) were chosen to determine their affinity and interphase formation with the epoxy system. The final concept involves the use of a thermoplastic as a boundary layer on top of the epoxy to establish a connection between two joined parts by resistance welding. The thermoplastic boundary layer was generated by co-curing during the curing process. The experiments resulted in a high compatibility between phenoxy and PMMA in the selected epoxy system which leads to excellent mechanical properties after welding. Phenoxy showed the strongest joints, with an average lap shear strength of 25 MPa, demonstrating the high potential of this joining technique for industrial applications in the automotive and wind energy fields.

2022-01-21, Zweifel, Lucian, Brauner, Christian, Teuwen, Julie, Dransfeld, Clemens

This study presents two novel methods for in situ characterization of the reaction-diffusion process during the co-curing of a polyetherimide thermoplastic interlayer with an epoxy-amine thermoset. The first method was based on hot stage experiments using a computer vision point tracker algorithm to detect and trace diffusion fronts, and the second method used space- and time-resolved Raman spectroscopy. Both approaches provided essential information, e.g., type of transport phenomena and diffusion rate. They can also be combined and serve to elucidate phenomena occurring during diffusion up to phase separation of the gradient interphase between the epoxy system and the thermoplastic. Accordingly, it was possible to distinguish reaction-diffusion mechanisms, describe the diffusivity of the present system and evaluate the usability of the above-mentioned methods.

2020-03-17, Brauner, Christian, Nakouzi, Sawsane, Zweifel, Lucian, Tresch, Jens

The primary objective of this study is to determine the interphase behaviour of a thermoset epoxy resin that is commercially used for carbon fibre–reinforced composite materials in aerospace structures and a suitable thermoplastic material that can be used as a boundary layer. The thermoplastic boundary layer will be used for welding purposes to join structural components with a fraction of the effort compared to conventional gluing processes. In this study, the interphase formation of an epoxy resin with several thermoplastic materials, namely, polyetheretherketone, polyvinylidenfluoride, polyphenylensulfide and polyetherimide (PEI), is studied via hot-stage microscope experiments. Based on this study, PEI was selected, and a detailed study was performed to determine the dependency of dissolution, diffusion and phase separation mechanisms under various isothermal conditions. Additionally, the welding behaviour was investigated by a resistance welding rig whereby the process parameters were statistically varied to optimize the lap shear strength. The results of this study will enable a statement about the interphase development, the morphology and the mechanical properties which is a key element of fully understanding the process.