PhD.Dissertation:Gizem Semra Arıtürk
A SUSTAINABLE APPROACH FOR THERMOPLASTIC COMPOSITES WITH TAILORABLE CHARACTERISTICS BY THERMOKINETIC HYBRIDIZATION OF WASTE CELLULOSE AND VERMICULITE
Gizem Semra Arıtürk
Materials Science and Nano Engineering, PhD Dissertation, 2024
Thesis Jury
Prof. Yusuf Ziya Menceloğlu (Thesis Advisor)
Prof. Fevzi Çakmak Cebeci
Assoc. Prof. Burcu Saner Okan
Prof. Cengiz Kaya
Assoc. Prof. Senem Seven Avaz
Date & Time: July 18, 2024 – 1PM
Place: FENS L029
Keywords : Hybridization, Polypropylene, Polylactic acid, Vermiculite, Cellulose, Upcycling, Sustainability
Abstract
The ever-increasing demand for polymer-based high-performance materials with reduced environmental impact has driven research towards more sustainable and greener composite material designs. The fundamental approach in such designs is using either natural or recyclable material types that ensure a more sustainable product life cycle while providing an equivalent material response with respect to their non-green alternatives. Such demand can only be achieved with a correct material selection process targeted towards enhanced constituent interactions and effective manufacturing strategies. The implementation of these processes would allow the discovery of novel reinforcement agents adaptable to conventional thermoplastic polymers.
The main aim of this thesis is to present the outcomes of composite hybridization effort where multiple reinforcement phases are introduced to a biodegradable (PLA) and a recyclable (PP) polymer matrix. Considered reinforcement types were cellulose fibers which is a textile industry waste and vermiculite which is a naturally abundant organoclay. The rationale behind reinforcement selection relies on the fibrous morphology of WC and plate-like morphology of VC. The synergistic effect of both reinforcement types with significantly different properties are searched for when manufactured by a thermo-kinetic mixing followed by injection molding.
The thesis reveals the constitutive interactions between WC/VC and host matrices that lead to improved mechanical response along with ease of processing. The fundamental outcome of the PLA-base hybrid is the in-situ exfoliation of VC under high shear mixing which turns VC into nano-metric platelets that both ensure effective stress transfer between main reinforcement phase that is fibrous WC and micro-crack deflection in brittle PLA matrix. The findings indicated that the integration of WC and VC into PLA can enhance the Young's modulus by up to 127% and the flexural modulus by up to 137%, while maintaining the tensile strength at an optimal level.
From thereon, the thesis focuses on the results achieved when similar hybridization effort is applied to ductile PP polymer with significantly different mechanical response. Presented results revealed the presence of a serious WC entanglement which governs the overall mechanical response and the inability of VC to form the PP/VC fibrils in the presence of WC. However, the mechanical response of the obtained hybrid composites was even more improved due to inherent ductility of PP. When used in the presence of high fiber content (20WC10VC) such platelets contributed significantly to tensile strength by making PP more brittle and allowing for an effective stress transfer during WC cluster debonding events. The enhancements in the PP hybrid composite were 118% for Young's modulus and 115% for flexural modulus. Furthermore, the strength of PP was increased by the hybridization of WC and VC. The flexural strength and tensile strength exhibited increases of up to 42% and 56%, respectively.
The nature of this in-situ exfoliation is further evaluated where the effect of polarity difference between PLA and PP on VC interlayer region that contains crystalline water molecules. By focusing solely on VC composites arrives an important conclusion which is the formation of PP/VC fibril structures in non-polar PP matrix. Such fibrils are found to be formed of VC platelets uniformly distributed in PP fibrils occurred under extreme shear. Study proves that solely VC can enhance the Young's modulus of PP composites by up 110% Contrarily it has been explored that although the presence of crystalline water causes a better in-situ exfoliation, it causes water evaporation from polymer surface. The study has demonstrated that the addition of VC can significantly enhance the Young's modulus of PLA composites, with an increase of up to 147%. Presented results underlined the necessity of the hybrid approach.
Consequently, this thesis explores the impact of polymer type on the interaction with the fillers. It investigates the use of WC and VC in both PLA and PP composites, analyzing the resulting morphology and its influence on mechanical properties. By combining bio-based materials and natural fillers with innovative processing techniques, this research paves the way for the development of high-performance, eco-friendly composites. This thesis presents valuable insights into the potential of these green composites for various industrial applications requiring improved mechanical properties without compromising environmental responsibility.