Valorizing and Remediating Synthetic Polymers with Tenacious Backbones

Abstract

Synthetic polymers or plastics are manufactured long-chained molecules primarily sourced from nonrenewable fossil fuels like petroleum. Owing to their superior properties (e.g., durability, impermeability, and high strength to mass ratio), plastics are the most widely used material, replacing traditional materials like metals, wood, and glass. However, the same properties that make plastics desirable are also responsible for their recycling difficulty and biopersistence. As a result, over 79% of the 6.3 billion metric tons of plastics waste generated to date accumulates in landfills or escapes into the environment, persisting for decades or centuries. This thesis highlights the immensity of plastics pollution and contributes polymer chemistry-based solutions in recycling and remediation methodologies. Chapter 1 emphasizes that the desirable properties of plastics are also responsible for their problematic persistence in the environment, causing a predicament where plastics are indispensable yet detrimental to the environment. Because it is impractical for society to avoid plastics altogether, developing recycling and remediation methods that use waste plastics as feedstocks alleviates their environmental impact. Many polymers, however, cannot be recycled via mechanical processes, and as a result, they are incinerated or landfilled. These waste plastics slowly disintegrate into microplastics, which are significant environmental pollutants and have problematic health effects. One example is the superabsorbent material made of crosslinked sodium polyacrylate used in diapers and feminine hygiene products. We aimed to develop a chemical recycling solution for the superabsorbent polymer used in Procter and Gamble’s (P&G’s) hygiene products (PAAP&G). After discussing various chemical recycling methods, we highlight the unavailability of closed-loop recycling methods for PAAP&G. We then transition to chapter 2, where we present an open-loop recycling method inspired by the common acrylic acid origin of PAAP&G and pressure-sensitive adhesives. This transformation was executed in three steps, namely (i) decrosslinking via hydrolysis, (ii) an optional chain-shortening step via sonication, and (iii) functionalizing via base-mediated esterification. Viscoelastic properties were tuned by adjusting the molar mass using sonication or incorporating amine functional groups, making adhesives spanning various applications, including tapes, bandages, and sticky notes. Any new recycling methodology requires an unbiased comparative evaluation to determine its processing and environmental impacts—chapter 3 utilized life cycle assessment to evaluate and improve the recycling method presented in chapter 2. The significant improvements to the previously developed process include: (i) replacing the base hydrolysis with acid hydrolysis and (ii) replacing the base-mediated esterification with Fischer esterification. Life cycle assessment suggested that our new approach outperforms the conventional petroleum-based route on nearly every metric, including carbon dioxide emissions and energy usage. Recycling diapers and feminine hygiene products through this method could avoid the disposal of 2 million metric tons of polymer waste annually. Chapter 4 begins with accidentally discovering that the adhesives developed in chapters 2 and 3 effectively captured micronized rubber, a type of microplastics, in a mixed solvent waste container. Next, we demonstrated the removal of various microplastics using adhesive-coated glass slides followed by quantitatively evaluating removal in suspensions of 10 μm polystyrene as a function of adhesive molar mass. We successfully quantified polystyrene removal using flow cytometry without fluorescent labeling and confirmed > 99% removal efficiencies. Chapter 5 summarizes each chapter and recommends future directions, including preliminary work towards developing a one-pot process to recycle PAAP&G to make PSAs where the idea stems from the similarity between acid hydrolysis and acid-catalyzed esterification.