CAREER: Bridging the Gaps Among Commodity Thermoplastics, Engineering Polymers and Thermosets via Thermally Reversible Crosslinking

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Over 70 billion pounds of plastic materials are used each year in the US, but less than 9% of them are recycled. This plastic waste is typically directed to landfills where it poses a threat to the health of the environment. A major reason for this low recycling rate is that these plastic materials are comprised of a variety of chemically distinct polymers that are used for different applications (e.g., commodity, thermosets, and engineering). These polymers are challenging to separate and cost-effectively recycle. For example, 20% of these plastic materials are thermosets, which cannot be recycled by melt-(re)processing methods. This CAREER project aims to investigate a new class of plastic materials (thermally reversible crosslinked polymers) that combines the best qualities of commodity thermoplastics, engineering polymers, and thermosets, while still maintaining their recyclability/melt-(re)processability. The feedstocks for these thermally reversible crosslinked polymers will be polyolefins. Polyolefins account for two-thirds of waste plastics, making them the most abundant and inexpensive plastic found in landfills. This new class of materials will divert discarded plastics from landfills into value-added products (e.g., automotive, aerospace, buildings and infrastructure, utility products, and transportation), thus reducing burdens on landfills and diminishing the leakage of plastics into the environment. The outcome of this proposal will strengthen the domestic (eco)-manufacturing, which will generate a multitude of societal and economic benefits. The research components of this project are integrated with several education and outreach activities that the investigator has developed to prepare a skilled workforce for the US manufacturing sector with a focus on enhancing the participation of women and minority groups in STEM programs. Polyolefins (e.g., polyethylene (PE), polypropylene (PP)) are the least-expensive plastics and account for two-thirds of all polymers produced. However, they also account for more than two-thirds of all plastic waste in landfills. This CAREER proposal aims to investigate and develop novel chemical modification methodologies that will transform discarded PE, PP, and their blends into thermally reversible crosslinked (TRC) polymers. These TRC-polymers will exhibit the desirable qualities of conventional thermosets (solvent resistance, dimensional stability) and engineering polymers (mechanical properties and creep resistance), while still offering comparable processability and recyclability to thermoplastics. Industry-relevant reactive extrusion processes will be used to create TRC-polyethylene (Objective 1), TRC-polypropylene (Objective 2), and TRC-polyolefins (Objective 3) from PE and PP feedstocks. The effects of numerous variables (e.g., melt-processing parameters, nature and amount of reversible crosslinkers, additives, catalysts) on the chemistry and performance of TRC-polymers will be investigated. Advanced spectroscopic, microscopic, and analytical methods will be used to characterize the molecular level chemistry of the TRC-polymers. The physico-mechanical properties such as thermal, solvent resistance, creep resistance, and tensile properties will be validated by standard testing protocols and are expected to be comparable or superior to those of commercial thermosets and engineering polymers. The outcome of this proposed work has the potential to transform the landscape of polymer science and eco-manufacturing by pioneering innovative methods for TRC-polymers. The new knowledge generated through this work will create novel avenues for research in advanced materials and will transform the fields of additive manufacturing, sustainable manufacturing, and offer unique feedstock materials for high-performance composites, foams, and melt-(re)processable interpenetrating networks. In addition, the development of a highly skilled future workforce is an integral part of this research which will be accomplished via training opportunities for undergraduate and graduate students, industry-academia collaboration, and creating an advanced curriculum. Outreach programs for K-12 students as well as training opportunities for high school teachers are planned to inspire young students towards STEM careers. A focus will be on the inclusion of programs that foster the participation of women and minorities in STEM programs. During this project, the future workforce will be educated and equipped to understand and appreciate emerging technological advances, thus strengthening the US sustainable manufacturing sector.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.