Date

2017

Author

Varsavaş, Sakine Deniz

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The purpose of the first part of this thesis was to investigate how optimum mechanical properties (strength-modulus-toughness) of inherently very brittle polylactide (PLA) could be obtained by reinforcing with E-glass fibers (GF) and blending with thermoplastic polyurethane elastomer (TPU). Composites and blends were compounded by twin-screw extruder melt mixing, while specimens were shaped by injection molding. SEM analyses revealed that 15 wt% GF reinforcements and 10 wt% TPU domains, alone or together, could be uniformly distributed in the PLA matrix leading to significant improvements in properties. Mechanical tests indicated that use of TPU blending alone resulted in enormous increases in the ductility and fracture toughness values, while GF reinforcements led to significant increases in strength and elastic modulus values. When GF and TPU were added together, it was observed that crack deflection, debonding and fiber pull-out toughening mechanisms of GF reinforcements were as effective as the rubber toughening mechanism of TPU blending. Additionally, DSC thermograms revealed that crystallinity amount of PLA would be increased almost two times due to especially heterogeneous nucleation site actions of GF reinforcements and fine sized TPU domains. In the second part of the thesis, the purpose was to compare performance of PLA based materials shaped by the traditional injection molding technique versus 3D-printing additive manufacturing. Comparisons were performed not only for neat PLA but also for its TPU blend and GF reinforced composites. Performance comparison of the injection molded and 3D-printed specimens were especially conducted to compare their mechanical properties (strength-modulus-toughness) by tensile, flexural and fracture toughness tests. Other comparisons such as their macro-level appearances, fracture surface morphology and thermal behavior were also performed by photographic images, SEM, DSC and TGA analysis. It can be concluded that use of 3D-printing in the shaping of neat PLA and PLA/TPU blend was generally very beneficial; on the other hand, due to the differences in the orientation of the GF reinforcements, there could be certain reductions in the mechanical performance of PLA/GF and PLA/TPU/GF composite specimens. The objective of the last part of this thesis was to explore the degree of improvement in the resistance of biodegradable PLA structure against atmospheric weathering (outdoor) conditions when reinforced with only 15 wt% GF. For this purpose, both neat PLA and PLA/GF composite specimens were exposed to accelerated weathering conditions of both UV-irradiation and moisture cycles in accordance with ISO 4892-3 standards for various periods till 400 hours. Many characterization techniques revealed that the alterations in the structure and properties of the specimens were due to the drastic decrease in the molecular weight of the PLA matrices via chain scission reactions. It was observed that reductions in the mechanical properties (strengthmodulus- toughness) of the neat PLA were much more critical compared to the reductions in the PLA/GF composite. For instance, the reduction in the tensile strength of the neat PLA specimen was as much as 92%; while that reduction for the PLA/GF specimen was only 34%. Because, inorganic strong glass structure of the GF reinforcements having almost no chemical degradation during weathering periods kept their actions in the composite strengthening-stiffening-toughening mechanisms. 

Subject Keywords

Polylactic acid., Glass fibers., Polyesters., Elastomers., Polyurethane elastomers., Three-dimensional printing.

URI

http://etd.lib.metu.edu.tr/upload/12621302/index.pdf
https://hdl.handle.net/11511/26678

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Graduate School of Natural and Applied Sciences, Thesis