Doctor of Philosophy
Mechanical and Materials Engineering
3D printing technology with valuable features, including cost-saving, easy access, and unlimited structure design, has attracted significant attention and been employed for production use. This technology has also been considered as a sustainable manufacturing method and quickly developed in recent years. However, the development of sustainable 3D printing is still facing challenges, especially in waste management. Thanks to the flexibility of 3D printing and diversified printing mechanisms, the big step forward can be approachable by the transformation from materials. This dissertation presents a variety of strategies designed for sustainable 3D printing development based on the combination of dynamic covalent chemistry and 3D printing technique. Dynamic covalent bonds provide polymers, including thermosets, with responsive covalent adaptable networks, allowing the materials to be reversible, leading to a recyclable material technology. Through synthesizing epoxy and polyurethane based dynamic covalent polymers as 3D printing materials, together with developing a corresponding 3D printing technique, the strength of green 3D printing is explored.
In this study, OH functionalized multi-walled carbon nanotubes (MWCNTs-OH) are incorporated in the printing material to improve the mechanical property and tailor the photothermal conversion capability of the developed materials. Due to the super high strength and minuscule size of MWCNTs, they are widely used to reinforce polymer as fillers. With MWCNTs-OH incorporating, the ultimate tensile strength and Young's modulus of samples both increased. In particular, young's modulus of 2 wt% DTDA-PU was about 8.6 times higher than the pure sample. Furthermore, MWCNTs are functioning as an excellent photothermal converter to allow the heat triggering to be replaced with a laser light. Therefore, near-infrared (NIR) laser source is utilized as the heat source for targeting the damaged spot of printing parts precisely for in-situ repair application. After the sample is cut in half and repaired, the mechanical properties recover to 86.3% after three times NIR laser-triggered in-site repair. In addition, a creative approach of contactless supporting structure removalis explored to promote post-processing automation and reduce printing parts defective rate. Our sustainable 3D printing strategy has developed an environmentally friendly and energy-efficient technology, which paves the way toward a circular economy.
Summary for Lay Audience
Sustainable manufacturing aims to reduce the consumption of finite natural resources and the negative impact on our environment. 3D printing technology, also known as additive manufacturing, is a process to fabricate 3D objects directly from a digital 3D model via computers. This technology can effectively reduce waste by avoiding conventional centralized mass productions. However, as 3D printing polymers being the major player in the current printing materials market, their waste disposal has caused an environmental burden. This work takes the further step of sustainable 3D printing development by developing the fully recyclable and repairable 3D printing technology.
Dynamic covalent bonds are the key to solving the problem. These are the covalent bonds that can exchange or switch between several molecules under certain conditions. Therefore, dynamic covalent bonds allow polymers, including thermosets, to be reversible, leading to recyclable and repairable materials technology. In this work, disulfide bonds and transesterification, as two of the most widely studied dynamic covalent bonds, were utilized for our recyclable and repairable printing materials development.
The relatively low mechanical property of pure polymers is another issue that limits the applications of 3D printing polymer products. Therefore, carbon nanotubes (CNTs), as the strongest and stiffest materials yet discovered, were incorporated in our recyclable and repairable printing materials to improve their mechanical performance. Furthermore, CNTs can be employed as light to heat convertors that can generate large amounts of heat by absorbing light. The stimulus to trigger the above dynamic covalent bonds is heat. However, heat energy is not easy to control, especially during the repair process for complex structures. The dynamic covalent polymers with CNTs incorporated can convert the stimulus from heat to light. Therefore, the repairing processes of damaged complex structures can be precisely and remotely operated with laser aiding. In addition, support structures are always necessary for 3D printing parts with overhang structures and need to be removed after printing. Traditional support removal processes by mechanical force are labor-intensive and prone to error. The laser-aid technique can effectively reduce the defective rate by achieving contactless removal, which can also reduce the waste of resources.
Zheng, Mingyue, "Study of Recyclable and Repairable Dynamic Covalent Polymers for Sustainable 3D Printing Development" (2021). Electronic Thesis and Dissertation Repository. 7673.
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