Development and optimization of selective laser sintered-composites and structures for functional applications
Polymer nanocomposite technology is an emerging field in which nanoscale fillers are embedded into a polymer to produce materials with advanced functionalities. The properties imparted by carbon nanotubes (CNTs) to the polymer can be remarkably good. Thermoplastic polymers such as polyamide 12 (P...
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Format: | Theses and Dissertations |
Language: | English |
Published: |
2018
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Online Access: | http://hdl.handle.net/10356/73195 |
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Institution: | Nanyang Technological University |
Language: | English |
Summary: | Polymer nanocomposite technology is an emerging field in which nanoscale fillers
are embedded into a polymer to produce materials with advanced functionalities. The
properties imparted by carbon nanotubes (CNTs) to the polymer can be remarkably good.
Thermoplastic polymers such as polyamide 12 (PA12) and thermoplastic polyurethane
(TPU) incorporated with CNTs have the potential to create a whole new generation of
composites with significantly improved performances through the selective laser
sintering (SLS) system instead of the conventional manufacturing processes of
nanocomposites. This PhD research aims to develop new polymeric nanocomposite
powders, optimize the process parameters of SLS for printing these powders, and
investigate the multi-functionality of the printed composite parts and their applications
in energy absorption and dissipation.
The development of composite powders is the primary step for the SLS process. A
surfactant-facilitated latex technique was developed to prepare multi-walled CNTs
(MWCNTs)-coated polymer powders, which possess the desirable microstructures and
surface morphologies for the SLS process. This method of embedding nanoparticles
into/onto polymeric powders was facile, green, low cost and scalable, providing a
universal route for the rational design and engineering of configuration and morphology
of composite powders. The thermal conductivity, thermal capacity, optical properties,
rheological and viscoelastic properties, and flowability of the new powders were
properly characterized for further evaluation.
These powder properties affect the powder deposition, bed temperature control andsintering performance of the entire laser sintering process. A methodology of process
optimization and powder evaluation was proposed, which applied a simplified
theoretical model to calculate the energy required for polymer melting and
decomposition and then predict the effective range of the input laser energy. This
methodology can effectively narrow down the working range of sintering parameters
and identify the set of optimal process parameters in the SLS process. Additionally, the
established model can be generally applied to the prediction of the effective range of the
input laser energy for semi-crystalline and amorphous polymers, such as PA12, TPU
and their composites.
To explore the end-use application of laser-sintered composite materials, their
multi-functionality was investigated, and the process-structure-property relationship
was studied to reveal the reinforcement mechanisms of nanofillers within the polymeric
matrix. The laser-sintered MWCNTs-reinforced composites exhibited promising
improvements in electrical conductivities and mechanical properties, as well as the
slight enhancements in thermal conductivities. The sintering of the MWCNTs-coated
polymeric powders could create the composites with segregated microstructures that
built the three-dimensional (3D) CNTs-network and formed the continuous conductive
pathways for electrons and phonons within polymer matrices. Meanwhile, after laser
sintering, the MWCNTs were retained at the powder-boundaries and prohibited the
movement of polymer chains upon mechanical forces, thus leading to the enhancements
in the mechanical properties.
With the improved mechanical toughness and strength, the CNT/PA12 composites
were proposed to be used for the fabrication of the 3D cellular and auxetic lattices for
the destructive energy absorption purpose. Another type of 3D soft auxetic lattices was
fabricated by the laser-sintered TPU material, which could be highly flexible and
recoverable upon cyclic compressive loading. Meanwhile, the energy absorption
capability of 3D lattices could be engineered by tuning the structure designs and
controlling the material formulations. Thus, the concept of the digitization and
integration of materials and structures was implemented into the SLS system and the
newly prepared polymeric nanocomposites was incorporated with complex 3D lattice
structures. The multi-scale and multi-functional architectures could be designed and
engineered to possess vibration resistance and impact shock absorption.
This PhD dissertation developed a systematic and integrated methodology for
material development, process optimization and structure design in the SLS process.
This methodology provides an effective guidance for formulating materials and
engineering their morphology to match the stringent requirements of the feeding
material for the SLS process. The designed 3D auxetic or cellular lattices can be
manufactured through the optimized system using the newly developed materials to
achieve the desirable performance of energy absorption or dissipation. |
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