Sustainability Centered Photoresin Design for 3D Printing Using Dynamic Covalent Chemistry

Contemporary society is and will be facing from now on, worrying environmental challenges that reflect the actions from current and previous generations with a lesser sense of environmental awareness. As science and technology advance, it is our responsibility to learn and do better to ensure we stop and prevent future damage to the earth, carrying on sustainability principles into every step we take forward. The design of new materials is especially important in today’s state of the world because physical tangible materials like plastic, represent the most visibly detrimental callings for action. Numerous sustainable alternatives from feedstocks to end-of-life management have been explored over the last couple of decades, which have paved the way for more eco- conscious design of plastics. Abiding by these sustainability centered strategies will avoid worsening the already alarming plastic crisis and will be a great starting point for future generations to move forward. As society develops and advances technologically, the design of new polymeric materials requires the consideration for compatibility with the latest technological developments, to make sure that these new materials could be candidates to replace existing non sustainable ones. In plastic manufacturing, 3D printing technologies show great promise for adoption as the default methods in the near future, based on their versatility, resolution, on demand production and sustainability attributes. 3D printing allows rapid prototyping, creation of complex parts, diverse applications, and democratization of manufacturing. In terms of sustainability, 3D- printing’s on demand production capabilities, significantly reduce the need for transportation, storage and waiting time, which overall reduces the carbon footprint of the final products. There are some challenges for ensuring that the products from 3D printing technologies do not represent a threat to the environment and these could be addressed through different actions in each step of the process. Selection of renewable feedstocks obtained through sustainable processes is required since the beginning design stages. Compatibility with the best resolution 3D printing technologies like vat photopolymerization 3D printing, will ensure the products perform to expectations, reducing the possibility of creating waste before their use. Vat photopolymerization 3D printing like stereolithography (SLA) or digital light projection (DLP), mostly produce thermosetting polymers which usually lack the ability to be recycled, and due to their thermal and chemical resistance, they could represent a threat to the environment after their end-of-life if not correctly disposed. To overcome this challenge, efforts to create vat photopolymerization printed thermosets, are to be accompanied with a sustainable end-of-life disposal mechanism in mind. Lately, disposal mechanisms like reprocessing, degrading and chemical recycling, have been made possible through incorporating the use of dynamic covalent chemistry (DCC) in the synthetic design. The research presented in this dissertation includes sustainability alternatives for every step of the photo-resin design process, to create materials with low environmental impact, compatible with DLP 3D printing. Chapter 1 provides background information on the current environmental challenges associated with plastics, as well as literature examples on how DCC has endowed 3D printed materials with smart properties and mechanical performance that make them competitive, as well as disposal possibilities to handle the materials at their end-of-life without representing a threat to the environment. Chapter 2 describes the design of five bio-based resins for DLP printing with self-healing capabilities through the use of DCC. Transimination exchange reactions were chosen as the dynamic reactions based on their excellent performance without requiring the addition of a catalyst. These five resins possess bio-based content, are DLP printable, show varied mechanical performances, have self-healing and reprocessability capabilities. Chapter 3 describes the design of three completely bio-based photoresins using monomers derived from lignin, and a crosslinker with beta-hydroxy moieties that allows transesterification reactions to occur with assistance of a catalyst. Through dynamic transesterification reactions the resulting DLP printed thermosets exhibit self-healing and one of the formulations can be readily reprocessed with above 70% recovery of the mechanical performance. A post processing annealing step, improves the mechanical strength of the materials, increasing the competitiveness of these bio- based materials with conventional oil derived alternatives. Chapter 4 describes the development of resins with 70 wt % bio-based content using lignin, vanillin, and soybean oil. Methacrylated lignin has multiple hydroxy moieties that can be activated with the use of a catalyst to perform transesterification exchanges. These dynamic behaviors allowed the thermosets to self-heal and be reprocessed lowering their environmental impact. Chapter 5 includes the design of two resin formulations polymerized through thiol-ene chemistry, which enables degradation and chemical recyclability of the resulting thermosets. These two thermosets were synthesized with bio-based feedstocks and posses imine moieties capable of performing transamination reactions for self-healing behaviors. This research aims to describe sustainable alternatives for every step of the process of designing new polymeric materials, competitive through their smart properties, mechanical performance and compatibility with DLP 3D printing.