Integration of flexible capacitive sensors in polymers for monitoring of polymerization and degradation processes.

• Applicants: Professor Dr.-Ing. Axel Herrmann; Professor Dr.-Ing. Walter Lang
• Subject Area: Microsystems ; Materials in Sintering Processes and Generative Manufacturing Processes
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271616017

Online-Process-Monitoring (OPM) has the potential to extend the area of application of fibre reinforced polymers by reducing processing times and costs while increasing the quality. It is important to monitor the consolidation process, especially the curing process, since it is not always homogenous in the whole fabricated part. Therefore, sensors should be integrated in the material to monitor the fabrication process inside the laminate. Hence, it is necessary that the sensors withstand the fabrication process and do not influence the functionality of the produced part. To fulfil this goal, a miniaturized flexible dielectric sensor was developed within the project “Integration of Flexible Capacitive Sensors in Polymers for Polymerization and Degradation Monitoring”. The sensors were embedded in carbon fibre reinforced thermosetting polymers and the curing process was successfully monitored. Furthermore, the influence of the sensor on the mechanical integrity of the produced part was investigated. Quasistatic tests were used to demonstrate that the sensor does not significantly degrade the mechanical properties of the material. Within the scope of this project, the curing of thermosets as matrix material was investigated. Beside thermosets, thermoplastics are used in the fibre reinforced polymer production. Thermoplastics have the advantage that the production process can be considerably faster. One important consideration in terms of industrial application is that it is possible to reduce the manufacturing cycle time by using thermoplastics. Furthermore they can be remelted, which allows for repairing and recycling defect parts. During the curing process of thermosets, the polymer chains crosslink and covalent bonds are created. On the contrary, the polymer chains of thermoplastics do not crosslink but they can be aligned to each other. The ratio between aligned and amorphous regions is called degree of crystallization. This degree determines the mechanical and thermal properties of the material and can be adjusted by varying the process parameters. Therefore, the degree of crystallization should be measured online during the fabrication process of the fibre reinforced polymers using dielectric sensors and optical fibres.The goal of the project is to monitor the crystallization of thermoplastics during the fabrication process of fibre reinforced plastic composites, especially to measure and control as accurate as possible the degree of crystallization and the load transfer point to determine the solidification point inside the material.In parallel to the miniaturized dielectric sensors, optical fibre sensors should also be embedded in order to cover a broader electromagnetic spectrum. The combination of both measurement principles increases the amount of information significantly, which helps determining a suitable model for the evaluation of the process.

• DFG Programme: Research Grants