PhD Dissertation: Oğuz Alp KURUCU
MECHANICAL RECYCLING OF POLYOLEFIN WASTE MIXTURES VIA ORGANOMETALLIC COMPLEXES
Oğuz Alp KURUCU
Materials Science and Nano Engineering, PhD Dissertation, 2025
Thesis Jury
Prof. Dr. Yusuf Ziya MENCELOĞLU (Thesis Advisor)
Assoc. Prof. Dr. Bekir DIZMAN
Assoc. Prof. Dr. Serkan ÜNAL
Prof. Dr. Cengiz KAYA
Assoc. Prof. Dr. Reza NOFAR
Date & Time: July 23rd, 2025 – 14:00 PM
Place: FENS L029
Keywords : Polyolefin, Mechanical Recycling, Impact resistance, Compatibilizer, Compatibilization, Polymer Blends, Organometallic Complexes
Abstract
Polyolefins, particularly polyethylene (PE) and polypropylene (PP), dominate the global plastics market due to their low cost, chemical resistance, and ease of processing. However, their immiscibility poses a major challenge in mechanical recycling, leading to phase-separated blends with inferior mechanical performance. Organometallic complexes present a promising alternative to conventional compatibilizers by enhancing interfacial interactions at low concentrations, offering an efficient and cost-effective solution for improving the quality of recycled blends. This thesis discovers the effect of organometallic complexes as compatibilizing agents to be employed in the recycling and re-use of LLDPE/PP waste blends to achieve superior impact resistance. The compatibilization candidates were grouped according to transition metal type (Ti or Zr) and to alkyl chain length (8 or 18). The last parameter was the weight fraction (0.05% to 1%) of compatibilizers choice that was tuned during high shear thermokinetic mixing. To evaluate the potential, a comprehensive set of characterization techniques is employed, including differential scanning calorimetry (DSC) for assessing polymer crystallinity, small-amplitude oscillatory shear (SAOS) analysis to extract the three characteristic relaxation times, and V-notched impact testing to quantify mechanical performance. Complementary fractographic analysis of the impact specimens provides conclusive morphological evidence that supports the findings obtained through thermal and rheological characterization. Zirconium-phosphate ester complexes demonstrated a non-reactive, dispersive compatibilization mechanism, improving impact resistance by up to 95%. Small-amplitude oscillatory shear (SAOS) analysis revealed intensified intermolecular interactions, interpreted through relaxation times (τ₀, τₑ, τd). Trade-offs between entanglement (τₑ↑) and chain mobility (τd↓) were observed with varying alkyl chain length and concentration. Thermal analysis indicated low crystallinity disparity between PE and PP phases 2.2% and 4% for C8-Zr and C18-Zr, respectively, suggesting improved compatibility. In contrast, titanium-phosphate ester complexes exhibited a reactive mechanism, preferentially promoting PE crystallization and forming skeletal wall morphologies. A mechanism-aware approach combining thermal analysis and multiscale fractography revealed features such as plastic zones, shear lips, and a toughening mechanism termed "wall hybridization." SAOS results confirmed enhanced molecular interactions and distinct network, interfacial, and structural rearrangements. A comparative analysis highlighted the differing compatibilization pathways of Ti- and Zr-based complexes, both offering effective, low-load solutions for immiscible blends. The findings demonstrate the potential of these organometallics as adaptable, low-cost compatibilizers aligned with circular economy principles.