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Browsing by Subject "Polyethylene"

Now showing 1 - 5 of 5
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    Developing a Predictive Model and Novel Imaging Technique for the Failure of Polyethylene Insulators
    (2019-08) Zoltek, Daniel
    Polyethylene is the most widespread polymer used in insulative cable housings due to its low cost, high chemical resistivity and low permeability to liquids and gases. This does not mean, however, that the material is not susceptible to failure under environmental working conditions. Many techniques for monitoring both chemical and physical changes have been developed, though no attempts have been made to integrate these findings. Here, we put forth a model for the failure of polyethylene cable housings under thermooxidative conditions. This model revealed an absence of data on the monitoring of polyethylene crystalline structure during the aging process, which in part controls the insulative properties of the polyethylene. Polyethylene films (30 µm) were aged at 110°C for 24-hour periods in an oven and carbonyl content, a common aging indicator, was monitored. An ATR-FTIR crystallinity monitoring technique was developed and revealed a 3-phase change of crystalline structure upon thermal aging. To better visualize the hypothesized pore formation in the polyethylene, which follows aging, EIS was used to saturate pores with gold nanoparticles before elemental analysis and imaging with SEM. Results suggest the existence of these pores and the ability for ions to penetrate the aged films.
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    Durability of high density polyethylene for potable hot water applications: crack propagation.
    (2012-09) Singh, Gyanender P.
    Polyethylene (PE) pipes, are used for water delivery, are susceptible to oxidation. As a result of oxidation PE becomes brittle and brittle pipes/tubes crack under the influence of tensile loads. These cracks initially propagate slowly and later on grow quickly becoming unstable. The focus of this study is slow crack growth in high density polyethylene (HDPE). Crack propagation experiments were conducted to determine the dependence of crack growth on degradation and stress levels. HDPE samples, with 0.3mm thickness, were exposed to 80°C chlorinated water (5-8 ppm) for up to 65 days. Thin samples were selected to ensure uniform degradation through the thickness. Although the brittleness of the polymer can be evaluated using strain-at-failure, the drawback of this method is that it destroys the sample. The Carbonyl Index (CI) obtained by Fourier Transform Infrared (FTIR) spectroscopy was established as a nondestructive measure of the degradation level. CI ranged from 35 to 93. A higher value of carbonyl index represents a greater extent of degradation. The relationship between CI and loss of mechanical performance was validated by strain-at-failure. Crack propagation tests were conducted were conducted on degraded polymer samples at constant load. The load (stress level) ranged from 5.1 to 9.2 MPa. In all 5 samples were tested. It was found that the crack propagation rate ranged from 6.31 x 10-10 to 1.26 x 10-2 m/s while the stress intensity factor ranged from 0.91 to 4.07 MPa√m. For a single degradation level, regardless of stress, the data when converted to log scale, and fit with the linear elastic fracture mechanics (LEFM) relationship = CKn. As the degradation increased the crack propagation rate increased such that all data were fit by the relationship = C(CI)Kn such that the exponential parameter ‘n’ was a constant for all the samples regardless of the level of degradation. The LEFM model fit to the data was best for moderate and high levels of degradation corresponding to CI of 55 and 90. Scanning Electron Microscopy (SEM) images show minimal deformation in the region around the crack tip, and ductile fibril stretching in the process zone. While the polymer had become brittle upon oxidation, there is local ductility in the process zone. An LEFM approach is typically applied to brittle materials, while the SEM results show that crack propagation is a combination of brittle and ductile behavior. Future studies should consider other modeling approaches that allow for ductile behavior in the process zone.
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    High Speed Photography for Manuscript "On the Method of Pulse-Heated Analysis of Solid Reactions (PHASR) for Polyolefin Pyrolysis"
    (2020-11-16) Dauenhauer, Paul; Mastalski, Isaac; Sidhu, Nathan; Zolghadr, Ali; hauer@umn.edu; Dauenhauer, Paul; Dauenhauer Laboratory - University of Minnesota
    The data contains high speed photography of high temperature (500 - 650 deg C) pyrolysis of polyolefin films including low-density polyethylene and polypropylene. This is in support of the publication entitled, "On the Method of Pulse-Heated Analysis of Solid Reactions (PHASR) for Polyolefin Pyrolysis."
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    Multiblock Copolymers for Compatibilizing and Recycling Polyesters and Polyolefins
    (2022-06) Peng, Xiayu
    Plastics are ubiquitous in our daily life, but the currently low recycling rate, only 9% in the United States, led to most plastic waste in landfills, energy conversion, and leaking into the environment. Due to the immiscibility between polymers, the melting process of mixed plastics usually yields blends with poor mechanical properties. Therefore, presorting is often required before mechanical recycling, which translated into the added cost. In addition, multi-component products, such as multilayer films used in food packaging are not recycled at all. To tackle this recycling challenge, effective compatibilizers are needed to decrease domain sizes, increase interfacial adhesion, and thus improve the mechanical properties of recycled blends. In this thesis, we will focus on using multiblock copolymers (MBCPs) as compatibilizers to assist recycling of mixed plastics, specifically polyethylene (PE) and poly(ethylene terephthalate) (PET).First, I successfully synthesized and implemented PET-PE MBCPs for use as both adhesive tie layers in-between PET/PE multilayer films and compatibilizers for PET/PE blends. As adhesive tie layers, the PET-PE MBCPs led to films exhibiting adhesive strength comparable to that of commercially available adhesives. As compatibilizers, PET/PE (80/20 wt%) blends containing as low as 0.5 wt% PET-PE MBCP were melt mixed to mimic recycling mixed plastic waste, and they were found to exhibit mechanical properties better than neat PET. To understand the mechanisms responsible for the outstanding performance of these MBCPs, I systematically investigated the role of molecular architecture on compatibilization and transport. It is found that MBCPs are more efficient as compatibilizers in PET/PE blends than triblock copolymers (TBCPs) with comparable total molecular weights. It is believed that MBCP could form trapped entanglements or co-crystallization with homopolymers and therefore strengthen the interfaces to achieve tough blends. In addition, MBCPs showed significantly faster transport kinetics than TBCPs to the interfaces during static annealing. In addition, I explored the possibility of incorporating other PET miscible polyesters into an MBCP structure with PE. Degradable aromatic polyesters derived from salicylic esters, poly(salicylic glycolide) (PSG) and poly(salicylic methyl glycolide) (PSMG), are chemical structurally similar to PET and therefore of great interest. The miscibility of binary blends of PET with PSG and PSMG was systematically investigated by thermal and optical analyses, and we found both PET/PSG and PET/PMG are miscible over the entire composition range. We conclude that the miscibility originates from specific weak interactions between the polymer pairs. This new experimental finding may provide opportunities for the development of other BCPs containing aromatic polyesters and PE as compatibilizers.
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    Plastic Biotransformation Technologies: Development of a Novel Environmental QPCR Assay for Polyethylene Terephthalate Hydrolase, and Isolation/Characterization of Polyethylene Degrading Fungi and Bacteria from Environmental Samples
    (2020-08) Wedin, Nelson
    Plastic production, use, and accumulation in the environment—including in the bodies of humans and other animals—have been increasing for decades and are a cause of growing global concern. Common plastic waste is generally considered to be non-biodegradable. In recent years, though, a growing assortment of bacteria and fungi capable of degrading a variety of common recalcitrant plastics have been identified. In general, the enzymes capable of depolymerizing long-chain hydrophobic plastic polymers are not well studied. However, Poly(ethylene) Terephthalate (PET) Hydrolase is well described in the literature and is thus a suitable target for molecular identification and quantification technologies. PET is the plastic polymer used in most plastic water bottles and in polyester fabric. The discovery of PET-degrading organisms and PET hydrolases is leading to the generation of biochemical technologies for the recycling and upcycling of PET, as well as the search for PET hydrolases that have greater activity on commercially relevant PET polymers. The incidence of PET hydrolase in metagenomes appears rare, though the quantification of PET hydrolases in environmental samples is unknown. Because plastic-biotransforming organisms are considered rare and slow growing, the process of isolating and characterizing these organisms is long and involved. This thesis presents two distinct, but interrelated, experimental trajectories related to the advancement of the study of plastic biotransformation. The first study focused on the molecular level, and the second study focused on microorganisms. In the first study, novel Quantitative Polymerase Chain Reaction (QPCR) primers were developed and tested for the ability to selectively amplify PET hydrolase genes from environmental samples. The products from these primers, used on eight environmental DNA extracts, were subjected to amplicon sequencing. Multiple sequence analysis methods confirmed the successful amplification of published PET hydrolase sequences, as well as sequences that show a high potential for being PET hydrolases. The on-target hit percentage and on-target hits varied substantially across samples, and this assay will require further optimization for specificity and quantification efficacy before it can be used for absolute quantification (i.e., gene copies/ ng DNA). There is reason to suggest that this assay can measure relative abundance of PET hydrolases, and thus relative genetic PET bioconversion potential. By providing comparative analysis that is both faster and less expensive than traditional techniques, this tool enables the rapid determination of ideal conditions to find and cultivate PET hydrolytic organisms. The core results of this analysis are presented in Figures 26, 28, and 29. In the second study, the focus was to enrich for and isolate (as individual species or consortia), identify, and evaluate microorganisms capable of Polyethylene (PE) biodegradation and biomineralization by culturing microbes in media where PE is the sole carbon source. Although the impact on the environment of PET (the polymer studied in the first study) is substantial, it pales in comparison to the impact of PE, which is used primarily for single-use items and is the most abundant type of plastic manufactured on the planet. Currently, no enzymes capable of degrading PE are well described, though some fungi and bacteria have been shown to degrade and utilize PE as a carbon source. In this set of experiments, preferential focus was given to fungi. Microbes that degrade and live on LDPE powder were enriched from environmental sources. A cogent argument for the confirmation of Low-Density PE (LDPE)-biodegrading organisms is presented from the limited data available (see below for limitations resulting from COVID-19 lockdown). LDPE-biodegradation can be seen in the isolate “Ath” (flask 6, a filamentous fungi that macroscopically appears to be Trichoderma sp.). The macroscopic observation of PE biotransformation for culture “Ath” is documented in Figure 33, where Flask 6 clearly shows modification to the PE powder. Modification increases with longer incubation and is not observed in the otherwise-identical non-inoculated control (Flask 34, Figure 33). Similar results are observed for other cultures, along with the growth of biomass and spore production. Thus, LDPE-biodegradation is also the most likely explanation for at least nine other environmental isolates. And microscopic confirmation of growth in this culture as well as others is presented in conditions where the only carbon source is PE powder. Both bacteria and fungi were shown to degrade the low molecular weight PE powder, though quantitative analysis on commercially relevant PE films was not completed. Tentative taxonomic hypotheses and the exciting possible implications of PE degradation within these taxa are presented, though genetic identification was also unable to be performed due to lockdowns. This research project was cut short prematurely due to mandatory laboratory lockdown in response to the COVID-19 pandemic. While both prongs of the studies described in this thesis were affected, the isolation and PE biodegradation assay was more seriously limited in that all quantitative analyses were unable to be performed. The discussion section reflects the limitations that resulted, as well as the adjustments that were made to compensate for these limitations.