| dc.description.abstract | The goal of the research provided in this dissertation is primarily focused on the solvent-based chemical recycling of multilayer plastic waste from food packaging. Additionally, elucidation of the crosslinking mechanism of ethylene vinyl alcohol (EVOH) under thermo-oxidative degradation was completed to further the understanding of its thermal processing and recyclability. The significant accumulation of global plastic waste year-by-year has motivated considerable interest in improved recycling technologies. The inability to individually reclaim all the constituents within multilayer plastics, especially in food packaging, greatly contributes to the increasing plastic waste generated annually. Multilayer plastics are notoriously difficult to efficiently separate and recycle using traditional thermomechanical techniques because of the chemical and physical differences amongst the constituent polymers, which generally lead to worsened properties. However, solvent-based chemical recycling strategies have been recently explored for multilayer plastics to extract the individual polymers from these wastes. Solvent-based chemical recycling technologies offer a new approach to mitigating plastic waste by taking advantage of the differences in thermodynamic solubilities. Using this principle, each constituent polymer in multilayer plastic systems is selectively dissolved and recovered by antisolvent precipitation prior to reprocessing by traditional thermomechanical means.
Multilayer food packaging is commonly made up of layers of polyolefins or polyethylene terephthalates (PET), tie layers, and one or more barrier layers. The outer and inner layers are usually made of polyethylene, PET, or other polyolefins. These layers are adjoined to tie layers which are responsible for the compatibilization of adjacent barrier layers to non-polar permeants. Typically, the innermost layer is the barrier layer. The barrier layer in food packaging is often EVOH or a nylon copolymer, both of which are extremely effective at preventing gas permeation, hence keeping food products fresh. Because of its high value and growing market demand, substantial recycling efforts have been investigated specifically to recover and reclaim EVOH.
The first portion of this work explored the thermo-oxidative degradation of EVOH by using time-resolved rheology and nuclear magnetic resonance (NMR) spectroscopy along with gel permeation chromatography (GPC) to identify the crosslinking mechanism during degradation. EVOH is extremely susceptible to degradation under heat and air which quickly changes its microstructure, properties, and resultant processing. Time-resolved rheology was used to understand the temporal dynamics, crosslinking, and degradation kinetics of EVOH under thermo-oxidative conditions to further the understanding of its processability in recycling applications. Three EVOH copolymers with ethylene contents of 27, 32, and 48 mole percent were used in the investigation. The storage moduli of the long chains (and large structures) greatly increased during the duration of the tests, increasing by several orders of magnitude. Additionally, the complex viscosity versus frequency was completely altered; the flow behavior was initially a Carreau fluid, which completely shifted to a power law fluid after ~200 minutes at 200 °C in air. This suggested a significant change in the flow mechanics and correspondingly, processability. The Han plots displayed a plateau of G’ after ten cycles of SAOS, which corresponded to a notable microstructural alteration. Additionally, a complete arrest of the relaxation process was observed from the Cole-Cole plots, where the initial relaxation behavior was near Maxwellian. Lastly, a novel model was developed to understand the change in the chain dynamics during time-resolved rheology which led to the conclusion, within the studied range, that copolymer content did not influence crosslinking kinetics. To elucidate the crosslinking mechanism which arose during thermo-oxidation, various NMR spectroscopy techniques were employed including 1H, DEPT-135, COSY, and HMQC NMR spectroscopy methods. It was found that after the thermo-oxidative treatment, the terminal gamma lactone moieties were ring-opened which led to crosslinking and increased the degree of polymerization as determined by DEPT-135 13C NMR spectroscopy. This led to a crosslinked network, which was also detected using time-resolved rheology and further verified by increases in molecular weight noted from GPC. From this work, a mechanism for the crosslinking and degradation of EVOH was determined.
The second portion of this work examined the selective extraction of EVOH from multilayer plastics used in food packaging. The sources of real food packaging waste used in this work included K-Cups and Dole fruit jars and bottles. This work set the foundation for the selective removal of EVOH from other polymers like polyolefins and tie layers in real multilayer plastic systems. A variety processing conditions were also explored for this portion of the work using dimethyl sulfoxide (DMSO) as the selective solvent, which preferentially removes EVOH from polyolefins. The effects of selective extraction on EVOH from both plastic waste sources (and compared against neat EVOH pellets) was also explored. Characterization of the extracted EVOH was completed and compared to neat EVOH, which included Fourier-transform infrared spectroscopy (FTIR), melt rheology, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The EVOH which was extracted from the waste sources were slightly altered compared to their neat counterparts in terms of its spectroscopic and thermal properties. There was a slight spectroscopic signature present for carbonyl structures after extraction. This indicated that a small fraction of the hydroxyl pendant groups were oxidized during the dissolution or precipitation processes. The thermal properties were effectively unchanged compared to their initial counterpart. More interestingly, the extracted EVOH’s rheological properties were shifted higher compared to neat EVOH grades, while maintaining a similar viscoelastic profile (i.e. storage modulus shifted higher but exhibited the same frequency dependence). From this work, a protocol was developed that displayed that EVOH can be selectively extracted from multilayer plastic waste from food packaging using DMSO without the need for the step-by-step dissolution approach proposed in literature.
The third and last portion of this work investigated the application of supercritical fluids as antisolvents for precipitating EVOH from solution as well as understanding the saponification reaction of the recovered EVOH. By using supercritical carbon dioxide (scCO2) as an antisolvent, the disadvantages of liquid antisolvent precipitation methods or temperature swings can be mitigated and greatly reduce energy costs and the need for advanced separation processes. It was found that scCO2 induced precipitation of EVOH out of glacial acetic acid (selective solvent) and appeared to change precipitation characteristics and phase behavior depending on EVOH concentration; for dilute systems, a fine dispersion of particles was created, whereas concentrated solutions formed solid pieces. Glacial acetic acid partially converted a fraction of hydroxyl groups into acetate groups upon heated dissolution in the carboxylic acid solvent. Because of this, saponification with aqueous sodium hydroxide (NaOH) and hot washing of the recovered EVOH was explored to reconvert the acetate groups into hydroxyl units. Based on FTIR spectroscopy, NMR spectroscopy, DSC, and TGA the saponification reaction was deemed successful and resulted in the nearly complete restoration of hydroxyl units. While this technology was primarily explored for solutions of neat EVOH, it also has great potential for real plastic waste products.
The culmination of this research provides a basis for solvent-based chemical recycling strategies focuses on the selective extraction techniques for barrier layer recovery. Specifically, this work sets the foundation for the recovery of EVOH from multilayer plastics from food packaging. Moreover, this dissertation has shown that supercritical CO2 is a promising antisolvent for EVOH dissolved in DMSO; CO2 and DMSO can be recycled as solvent and antisolvent streams after precipitation using a pressure swing. Lastly, the thermo-oxidative degradation mechanism of EVOH has been elucidated, which is imperative for understanding how it can be thermomechanically reprocessed after extraction from multilayer plastic systems. | en_US |
