Browsing by Author "Yavuz, Zelal"

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Open Access
Effects of thermoplastic coating on interfacial interactions in advanced engineering composites for aerospace applications
(2023-05-03) Yavuz, Zelal; Khaligh, Aisan; Öz, Yahya; Tuncel, Dönüş
Delamination due to an inferior adhesion between reinforcement material and matrix in carbon fiber-reinforced thermoplastic (CFRTP) composites is a crucial problem to be solved. To this end, this study aims to overcome poor wettability between reinforcing phase, i.e., carbon fiber (CF), and thermoplastic matrix, i.e., polyetherether ketone (PEEK). Herein, CF’s surface was tailored by application of different polymeric sizing agents which have different chemical structures. Morphology and topology analyses were performed by Scanning Electron Microscope and 3D laser scanning, respectively. Later, a variety of wettability results were obtained by the sessile drop method used in Contact Angle (CA) measurements for CFs throughout application of each sizing agent applied by dip coating. Sizing materials were designed such that the chemical structure of CF’s surface could exhibit compatibility with the matrix itself. Consequently, complete wettability (CA: 0°) was achieved for CFs sized by HPEEK (CF/hydroxylated PEEK (HPEEK)) and the surface free energy (SFE) of CF was enhanced from 5.43 to 72.8 mJ/m2 while the SFE of the PEEK matrix is 40.1 mJ/m2. Moreover, sizing by HPEEK improved the average surface roughness of CF by 32% which enables optimized adhesion. Afterward, repetitive tensile tests were carried out to observe effects of improved interfacial interlocking on the mechanical properties of the final CFRTP composite. Stress–strain curves revealed that the tensile strength of CFRTP improved from 473 to 508 MPa through the sizing of CF by HPEEK whereas pristine PEEK has a much smaller tensile strength (98 MPa) than the aforementioned CF-reinforced composites.
Open Access
Investigation and control of the static electrification in polypropylene
(2016-08) Yavuz, Zelal
The electrostatic charging of polymers due to friction is such a common phenomenon in daily usage of polymers that can be a problematic issue for various applications, such as in electronic devices, textile, space industry and so on. Hence, understanding and controlling of the mechanism behind the static electrification, which is basically because of the charge accumulation on the material, is an important subject in terms of the applications. In the way to understand static electrification of bulk materials, examining the physical and morphological properties is crucial. On the other hand, when the physical properties are considered, the structure of polymer plays a significant role, yet there is a lack of knowledge in the literature about the relation between these structural properties and triboelectricity. As a reason of this, it can be pointed out that in the proposed mechanisms about the frictional electrification the structure-property relation could not get sufficient attention so far. In this thesis, the crystalline structure of polymer, which plays a crucial role in the determination of physical properties of polymeric materials, was studied and by using different treatment techniques, such as microwave radiation and mechanical stress, and the relation between the degree of crystallinity and triboelectric charging was investigated. Due to its economical cost and heat-sensitive degree of crystallinity that can be changed in a significant way polypropylene (PP) which is a semi-crystalline polymer was used in this study. Hence, by utilizing different spectroscopic and microscopic techniques the relation between physical properties and triboelectrification of polypropylene was investigated in detail. In order to understand the physical and chemical changes taking place in untreated and treated polypropylene X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Differential Scanning Calorimetry (DSC) are the techniques that were employed. In this study, by considering the mechanism behind static electrification the potential link between electrification and degree of crystallinity was designated. Furthermore, the generation of charge on the surface of mechanically treated polypropylene film was observed for the first time by this current work. The results lead to the fact that it is possible to convert mechanical energy into electrical energy without any contact between the objects by introducing physical forces onto the insulating materials and the reasons behind non-contact electrification was investigated. Therefore, in the light of the results obtained from this study, more efficient triboelectric generators can be designed to harvest electrical energy from mechanical energy.
Open Access
The investigation of advanced thermoplastic composite materials in aerospace applications
(2023-05) Yavuz, Zelal
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.
Open Access
Investigation of sizing materials for carbon fiber reinforced thermoplastic composites
(Sage Publications Ltd., 2024-09-18) Yavuz, Zelal; Öz, Yahya; Ece, Remzi Ecmel; Özturk, Fahrettin
For the manufacturing of mechanically strong and lightweight composite aerostructures reinforcement materials (e.g. carbon fibers, CFs) are the most convenient way. Therefore, sizing of carbon fibers is crucial for guiding them into service by protecting the CF’s surface. In this study, a novel sizing agent was developed and effects of this sizing on CFs’ physicochemical as well as surface properties were investigated. The impact on the fiber-matrix interphase behavior was analyzed. Results reveal that the surface free energy of CF was increased from 5.67 mJ/m2 to 13.13 mJ/m2 through sizing by enhancing the wettability property of CF. In addition, surface topography analyses indicate that the surface roughness Ra is 3.70 ± 2.59 nm for neat CF; 1.01 ± 0.65 nm for Polyetherimide (PEI) sized CF; and 1.71 ± 1.14 nm for PEI-Polyether ether ketone (PEEK) sized CF. Finally, it was concluded that an increment in the wettability can be related with chemical changes on the fiber’s surface.