| Summary: | Polymers and their composites are widely used in modern industries. They are of great interest in additive manufacturing (AM) due to their vast potential for various applications, especially in the medical, aerospace, and automotive industries. Currently, the most widely used materials for selective laser sintering (SLS) are engineering polymers based on polyamides, with polyamide 12 making up ~90% of the SLS material market. This Ph.D. research aims to investigate the use of polypropylene (PP) and its nanocomposite powders as a more practical alternative for applications where conventional polyamide materials for SLS are not optimal in terms of the trade-off between cost and desired properties.
An in-depth investigation of SLS of PP powders is fundamental to developing PP nanocomposite powders for SLS. Isotactic PP homopolymer and copolymer powders were comparatively investigated for SLS, and the processability of the polymer powders was evaluated in terms of powder morphology, powder flowability, melting behavior, and crystallization kinetics. An isothermal differential scanning calorimetry testing protocol was employed as an effective and efficient method to determine a suitable powder bed temperature. It was established that for PP with high regio- and stereo-irregularities, SLS serves as a viable alternative manufacturing technique for producing PP parts comprising γ-phase crystals that are difficult to be manufactured by conventional molding processes.
PP composites are widely used in the traditional plastics industry; in particular, organically modified montmorillonite (OMMT) is a nanofiller known for significantly enhancing the thermal and barrier properties of PP at low loadings. OMMT/PP nanocomposite powders for SLS were first developed on a small scale using four powder preparation methods, including melt extrusion combined with cryogenic grinding, thermally induced phase separation (TIPS), surface coating, and mechanical mixing. Evaluations were conducted on the shape and size, powder-bed surface roughness, and thermal behavior of the developed nanocomposite powders, as well as the mechanical properties of their injection molded tensile specimens. TIPS could produce powders with reduced crystallization temperatures and increased melt enthalpies that were beneficial for SLS and powder morphology ideal for SLS without post-processing. At low loadings, nanofillers with sheet-like structures such as OMMT could limit the increase in crystallization temperatures of the nanocomposite powder, thus ensuring their wide sintering windows for SLS. Nevertheless, melt extrusion proved to be the most favorable as the powders exhibited the highest thermo-oxidative stability.
OMMT/PP nanocomposite powders were thus produced at a large scale via melt extrusion and cryogenic grinding, and the properties of the powders that were critical to SLS were evaluated. The nanocomposite powders demonstrated significant widening of their sintering windows that greatly enhanced their SLS processability, and their laser-sintered specimens exhibited promising improvements in thermal stability. The developed nanocomposite powders demonstrated the potential of using OMMT to improve the SLS processability of PP and possibly other polymers by widening their sintering windows and reducing their melting range values without significantly lowering their mechanical properties. A wider range of powder bed temperature values could then be used for the nanocomposites, allowing for greater freedom in the designing and optimizing of SLS process parameters.
This Ph.D. thesis comparatively investigated PP homopolymer and copolymer powders for SLS, developed a systematic approach to the formulation of PP nanocomposite powders that could satisfy the stringent SLS material requirements, including the thermal and rheological properties, and established a method to effectively and efficiently determine a suitable powder bed temperature. This thesis would provide an effective guide for formulating PP materials for SLS and could serve as a foundation for future studies on PP and its composites for SLS.
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