The investigation of advanced thermoplastic composite materials in aerospace applications

Özensoy, Emrah
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Bilkent University
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The development of load-carrier reinforced composites is crucial in terms of a wide range of applications, such as aerospace, automotive, sports industry and so on. When these fields are taken into consideration, reducing the excessive weight of structural materials without any sacrifice in the performance is required. Thus, using reinforcement materials (e.g. carbon fibers) for polymeric matrices in composites is the most convenient way to follow. In this study, carbon fiber (CF) was used as a reinforcement material for thermoplastic based composites. Since bare CFs are too fragile to process they must be coated / sized such that the brittleness of CFs can be avoided during industrial applications. Therefore, sizing of carbon fibers is crucial for guiding them into service by protecting the CF’s surface. Yet, the traditional sizing agent (i.e. epoxy) is not suitable for handling continuous CF reinforced thermoplastic composites (CFRTPs) with high processing temperatures above 300 ℃. In this study a novel sizing agent was developed for this purpose. The effects of this sizing on the CFs’ physicochemical as well as surface properties were investigated. As a result, the impact on fiber-matrix interphase behavior can be analyzed. Moreover, the main problem for thermoplastic based composites which is the delamination between the reinforced fiber and thermoplastic matrix can be solved throughout the development of novel coating material so that inert carbon fibers can be made compatible with the matrix. In this thesis, the activation of carbon fiber’s surface, then providing a functional sizing agent and method in order to expel the present voids because of incompatibility between CF and thermoplastic matrix (i.e. Polyetherether ketone) were carried out by enhancing the adhesion. Hence, the wettability of CF by polyetherether ketone (PEEK) matrix was enhanced by altering the surface free energy of CF resulting in optimized adhesion. Thus, the delamination problem in thermoplastic based composites was solved throughout the sizing of CFs. The first part of this work consists of the elimination of current epoxy coating on the aerospace grade commercial carbon fibers. Then, the application of surface activation method was performed by acidic modification to make CFs ready for sizing process. The formation of functional groups (-COOH, -OH) on CF’s surface was achieved after degrading of present epoxy coating throughout CFs. As a result, the developed sizing agents could be binded easily onto CF’s activated surface through the hydrogen bonding. In the second part, four different polymeric sizing agents were prepared by taking the chemical compatibility with the matrix into consideration. The sizing process was performed via dip-coating method for the surface-activated CFs. The chemical and physical analyses for neat and treated CFs were carried out via microscopic and spectroscopic techniques. As a result of sizing process, the enhanced compatibility between the matrix and reinforcement material was proved by the Contact Angle Analysis and surface free energy calculations according to Young’s equation.

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Adhesion, Carbon fibers, Composites, Delamination, Polyetherether Ketone (PEEK), Thermoplastics
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