Cevdet Kaynak

The main purpose of this study was to investigate the effects of 10, 20, and 40 wt% micron-sized particles (aluminum nitride, aluminum oxide, silicon nitride, and silicon oxide) on the thermal performance of an epoxy resin used in microelectronic devices. Specimens were produced via a solution mixing technique followed by molding and curing. Although there were slight differences between the particle types used, various thermal analyses revealed that increasing the amount of all particle types significantly improved the thermal performance of the epoxy resin. The property that influences the thermal performance of microelectronic devices the most is thermal conductivity (λ). Heat produced during operation should be released via heat diffusion, which requires a certain level of λ. In this study, the use of a 40 wt% particle content increased the thermal conductivity (λ) by more than 3 times compared to neat epoxy (0.15 W/m·K). Another significant problem during the operation of these devices is the formation of “thermal strain mismatch” due to the different thermal expansion coefficients (α) of the materials used in the device that might lead to a loss of dimensional stability and malfunctioning. In this study, a particle content of 40 wt% decreased the thermal expansion coefficient of epoxy (49 × 10⁻⁶/K) down to 28 × 10⁻⁶/K, a decrease of −43%. Thermal performance also depends on the Glass Transition Temperature (T_g) values. In this study, a particle content of 40 wt% increased the T_g from 51 °C (neat epoxy) to 68 °C, an increase of 17 °C, and increased the Thermal Degradation Temperature (T_d) from 324 °C (neat epoxy) to 356 °C, an increase of 32 °C. Moreover, it was also revealed that there was no decrease in the lap shear adhesion strength of the epoxy resin after incorporation of any of the particle types. Additionally, the particles also increased the mechanical rigidity of the epoxy in terms of Storage Modulus at 25 °C and 50 °C.

In the first step of this study, purpose was to obtain and characterize cellulose nanocrystal (CNC) particles by applying sulfuric acid hydrolysis method to the starting material of microcrystalline cellulose (MCC) fibrils. After obtaining CNC particles via acid hydrolysis procedure, various analyses (SEM, DLS, FTIR, XRD, TGA) conducted revealed that average size of round shaped CNC particles was 38 nm with −30.4 mV Zeta potential value. They had monoclinic Cellulose-I crystal structure with Crystallinity Index of 80.6% and Crystallite Size of 3.39 nm. Their maximum thermal degradation temperature was 307°C with 23 wt% residue at 800°C. In the second step, the main aim was to investigate contribution of these obtained CNC particles when they were used as nano-reinforcement in polylactide (PLA) matrix biocomposites produced by industrially compatible melt mixing and shaping techniques. Mechanical tests revealed that when only 1 wt% CNC particles were incorporated into PLA matrix, increases in flexural strength and modulus were 29% and 51%; while increases in K _IC and G _IC fracture toughness values were 42% and 105%, respectively. Because, high degree of hydroxyl groups and presence of sulfate half-ester groups on the surfaces of CNC particles improved the interfacial interactions between the matrix and nano-reinforcement phase, leading to efficiency in strengthening and toughening mechanisms.

The main purpose of this study was to use “green materials” approach by investigating effects of only 1 wt% Cellulose Nanofibrils (CNF) on the strengthening and toughening of neat and blended polylactide (PLA) biopolymer matrix. For this purpose, first of all effects of CNF were investigated in PLA/CNF biocomposite specimens. After blending of PLA with 10 phr bio-based thermoplastic polyester (b-TPE) elastomer, effects of CNF were investigated also for this PLA/b-TPE/CNF ternary biocomposite specimens. Mechanical tests revealed that due to the efficient strengthening and toughening mechanisms, CNF increased flexural strength of PLA by 33%, while b-TPE increased fracture toughness of PLA by 104%. When CNF and b-TPE were incorporated together, synergism in the strength and toughness values were occurred. All bioblend and biocomposite specimens were produced by using the same “melt mixing” technique in a laboratory size twin-screw extruder, and their test specimens were shaped by conventional “compression molding”. Since shaping by “3D-printing” is frequently used in the biomedical sectors, another distinctive aim of this study was to reveal whether there were any differences in the strength and toughness values of specimens after their 3D-printing. It was observed that due to the “textured” structure of 3D-printed specimens, their flexural strength values were approximately 20% lower, while fracture toughness values were approximately 20% higher.

The main purpose of this study was to investigate effects of various electrospinning parameters on the morphology and diameter of cellulose nanofibril (CNF) filled polylactide (PLA) nanofibers. For this purpose, first of all effects of three important electrospinning parameters; polymer solution concentration, solution feeding rate and collector distance to feeding tip were studied. Then, effects of using higher amount of CNF, effects of using cellulose nanocrystal (CNC) particles, and effects of adding potassium chloride salt were also investigated. It was observed that when optimum electrospinning parameters were determined, then it was possible to obtain almost “bead-free” morphology and “finest” average diameter of 232 nm for PLA/CNF electrospun fibers. Increasing values of feeding rate and collector distance parameters resulted in bead formation and thicker diameters. On the other hand, increasing CNF amount, using CNC particles and adding KCl salt, all resulted in further decreases in the diameter down to 152 nm; mainly due to increased charge density of the polymer solution. Moreover, in vitro degradation analysis of all types of electrospun nanofiber mats in a simulated body fluid revealed that increasing the immersion period increased their degradation rate in terms of “% weight loss”. It was also observed that, mats with fine diameter fibers had higher degradation rate.

Case bonded solid rocket motors hold significant importance in both the defense and aerospace industries which typically consist of an insulating layer, a solid propellant, and a liner layer serving as an adhesive between the insulator and solid propellant. In this investigation, the aim was to assess the critical mechanical properties of five different HTPB based liner formulations under ambient conditions, as well as at elevated and reduced temperatures. The liner formulations incorporated micro carbon black, nano carbon black, nano silica, and nano titania as fillers. A comprehensive characterization study encompassing mechanical and thermal properties was conducted to evaluate the performance of these potential rocket motor liners. Mobility reduction of polymer chains and bond strengthening at lower temperatures led to the enhancement in mechanical properties of all filled and unfilled liners with decline in temperature. All mechanical properties of filled liner formulations increased significantly compared to unfilled formulation at all temperatures. Enhancements in mechanical properties were more pronounced with carbon blacks as fillers. Thermal analyses of each formulation revealed that there were improvements in the thermal properties of filled liners compared to unfilled one.

The main purpose of this study was to observe the degree of the improvement on the film adhesive joining performance of Poly(phenylene sulfide)/Carbon Fiber (PPS/CF) thermoplastic composite laminates when their surfaces were treated by atmospheric plasma technique. For this purpose, plasma treated surfaces were compared with untreated and the traditional grit-blasted surfaces. Treated surfaces were characterized by various techniques including contact angle, surface energy, surface roughness, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. Then, joining performance of the PPS/CF composite samples bonded by an epoxy-based film adhesive was determined by using three different mechanical tests; single-lap shear strength, mode-I interlaminar fracture toughness energy, and drop-weight impact toughness. Grit-blasting surface treatment increased surface roughness of the specimens enormously (7 times more), leading to mechanical interlocking as the dominant bonding mechanism. Contrarily, plasma surface treatment resulted in formation of chemically reactive sites, thus dominant bonding mechanisms in the film adhesive joining were certain polar interactions and chemical bonding. Mechanical tests pointed out that compared to grit-blasting, plasma surface treatment resulted in considerably higher joint performance regarding interlaminar shear strength (35% more), interlaminar fracture toughness (67% more) and impact toughness (28% more), with the cohesive failure mode. Accordingly, it could be stated that traditional grit-blasting could be replaced with plasma surface treatment applied before film adhesive joining of PPS/CF composite parts in aircraft industry.

For Poly(phenylene sulfide)/Carbon Fiber (PPS/CF) thermoplastic composite laminates, the main purpose of this study was to evaluate effects of “laser” and “plasma” surface treatments on the adhesion performance of primer painting by comparing with “untreated” and traditional “sandblasted” samples. For this purpose, after surface characterization of untreated and treated samples by Wettability, SEM, Surface Roughness, XPS and FTIR analyses; adhesion characteristics of an industrial primer paint applied onto PPS/CF surfaces were evaluated by using two different industrial methods: “Cross-cut Adhesion” tests and “Three-point Bending Adhesion” tests. Compared to the Untreated samples, it was observed that all surface treated samples could be assigned with the “best” primer adhesion grade of GT0. Moreover, surface treated samples had at least two times more maximum “Separation Load” with “cohesive” separation mechanism of primer paint layers. The success of the Sandblasted samples was due to efficient mechanical interlocking mechanism. For the Laser treated samples, adhesion performance was partly due to the mechanical interlocking and partly due to the chemical interactions; while for the Plasma treated samples it was mainly chemical bonding mechanism.

The main purpose of this study is to investigate usability of resistance welding joining method for poly (aryletherketone) (PAEK) thermoplastic matrix/carbon fiber (CF) reinforced composite laminates (PAEK/CF). For this purpose, effects of six different Interlayer Forms having different stainless steel meshes as Heating Elements and different woven glass fiber forms as Insulating Layers were studied. After determining welding parameters for each specimen group, performance of the resistance welding operations was compared by ultrasonic inspection, microscopic examination, DSC analyses, and by three different interlaminar mechanical tests. Analyses and tests generally revealed that use of stainless steel meshes as Heating Elements could supply proper amperage level for the heating, melting and crystallization stages of the PAEK matrix during welding consolidation. Depending on the lay-up configuration of Interlayer Forms, “amperage levels” determined for the heating, melting dwell and crystallization dwell were in the ranges of 46–100 A, 40–90 A and 25–50 A, respectively, It was also observed that in order to prevent Current Leakage problem to the other layers in the composite laminate, rather thicker and heavier woven glass fiber forms as Insulating Layers should be used. Otherwise, not only voids could form in the thermoplastic matrix, but also delamination might occur in the upper or lower PAEK/CF composite laminates being welded. When thicker and heavier insulating layer was used; interlaminar mechanical properties of Single Lap Shear Strength, Fracture Toughness under Mode-I and Mode-II were determined as high as 36.77 MPa, 3.76 kJ/m² and 4.71 kJ/m², respectively.

The main purpose of this study is to evaluate the effects of certain processing parameters on the mechanical performance of carbon/epoxy towpreg wound composite structures. For this purpose, composite sample productions and their evaluations were conducted in two steps. In the first step, dry winding of carbon/epoxy towpregs was used to produce flat composite plates. Their evaluation was performed by rheological analysis, interlaminar shear tests, and unidirectional tensile tests. In the second step, towpreg dry winding was used to produce composite pressure vessel samples. Their performance was evaluated by observing the effects of various winding process parameters on the safety of the vessels via hydrostatic burst pressure tests. Compared to the traditional wet filament winding, the main difficulty observed was maintaining the “straight towpreg path” necessary for efficient winding operations. This problem was prevented by applying higher tension forces during dry winding. Evaluation of the hydrostatic burst tests in terms of burst pressure, hoop strain and safe failure mode revealed that the optimum pressure vessel performance could be obtained in the vessel samples with “helical-hoop-helical winding layer sequence.” On the other hand, use of “complex helical pattern” resulted in no advantages at all, due to basically higher number of undulation zones acting as stress concentration zones.

The first purpose of this study was, as the first time in the literature, to investigate usability of polylactide (PLA) biopolymer as the matrix of high-performance composite laminated structures having woven forms of glass and carbon fibers. For this purpose, 2-14 layers of 2 × 2 twill Carbon Fiber (CF) and 1 × 1 plain weave Glass Fiber (GF) forms were stacked by PLA powders followed by consolidation of these layers by compression molding technique. After conducting various tests and analysis it was observed that significant improvements in the mechanical and thermal properties could be obtained by laminating PLA with woven CF and GF layers. For instance, mechanical properties obtained for PLA/CF laminates with 14 layers were flexural strength of 641 MPa, flexural modulus of 34 GPa and interlaminar shear strength of 38 MPa. Another purpose of this study was to compare mechanical performance of the PLA matrix laminates with Epoxy matrix laminates having exactly the same type and number of GF and CF layers. It was revealed that mechanical properties of the PLA/CF laminates were approaching to the values of the Epoxy/CF laminates (e.g. flexural strength of 607 and 673 MPa, respectively); while due to poor adhesion between the PLA matrix and Glass Fiber surfaces, the properties were lower in the PLA/GF laminates (e.g. flexural strength of 302 and 553 MPa, respectively). Therefore, it could be generally concluded that, in terms of mechanical performance; traditional thermoset Epoxy matrix could be replaced with the renewable biopolymer PLA matrix in the Woven Carbon Fiber laminates. For the Woven Glass Fiber laminates, a proper sizing treatment with a PLA compatible silane coupling agent would be necessary.