Motivation
Thermosets and thermoplastics are considered two separate categories of plastics, each with their own advantages and disadvantages and corresponding areas of application. Thermosets are mainly used in the structural area as infiltration resins for fiber composites. Thermoplastics are also used in fiber composites but are also used filled or unfilled directly as structural components.
In aerospace, thermoset composites are mainly used as structural materials for reasons of lightweight construction and temperature requirements. However, these materials are not weldable, have lower impact resistance, and are often less stable to chemicals. In these areas, thermoplastic would work very well.
Bringing the advantages of both classes of materials together is the basis of research on so-called interphases. The research is concerned with the formation, microscopic and macroscopic properties, mechanical characteristics and application areas of these hybrid materials.
In the Institute of Lightweight Structures, various material pairings are investigated with regard to different areas of application. However, we want to allow a wider range of possible usage parameters in order to also find solutions for applications that are not in the area of primary structures. In particular, we are concerned here with the areas of amorphous "engineering plastics" such as PSU or PC, which have so far been little researched in terms of their effect in interphases. Due to their lower melting temperature, these materials offer a broader range of applications in the field of weldability without damaging the underlying thermoset matrix.
Procedure
Elaboration of process limits for interphase formation
- DSC investigations of various thermoplastics
- Microscopic reaction investigations
- Establishment of process specifications for autoclave processes
Optical evaluation of interphase formation
- Micrographs and SEM examinations
- Measurement of formed interphases
- Analysis of the diffusion character
Determination of mechanical properties in comparison to bonded joints
- Single lap shear tests to determine the bond properties
- Correlation of material pairings and interphase shape with mechanical properties
- Comparison of interphase joints with bonded joints
Investigations for impact resistance
- Impact tests with different TP films
- Variation of layer thicknesses
- Development of technical scenarios
Investigations for weldability
- Development of welding parameters
- Investigation of different welding approaches
- Characterization of joint properties
- Implementation in additive manufacturing processes
Investigations for repair applications
- Development of basic process mechanisms
- Optical and mechanical examination of repair areas
Outlook
Interphases offer a very good opportunity to combine the various useful properties of plastics. In the subject area, mainly materials for structural primary structures have been investigated so far, leaving potential by the wayside. For secondary structures in aerospace but also applications in other technical fields, interphases can offer a possibility for better recycling, simpler process control, the elimination of auxiliary materials for compounds and general functionalization.
The aim is to identify all the available material pairings and to derive basic mechanisms for how thermosets and thermoplastics react with each other. In the project described here, as well as in many other projects, only a few pairings have ever been identified. Nevertheless, there are a large number of indicators that show what influences various parameters have on the formation of interphases. Based on this, further possible pairings should be worked out and made technically usable.
