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Browsing by Subject "polyethylene"

Now showing 1 - 6 of 6
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    Determination of Low Density Polyethylene Water Permeability, Transport Activation Energy, and Mechanical Properties after Thermal Oxidation and Immersion in Water
    (2019-08) Munir, Noumon
    Low Density Polyethylene (LDPE) thin film was exposed to oxygen gas overpressure and water. in a Parr Instruments pressure vessel at four temperatures 22, 50, 70 and 80°C; and, at 30 and 50 psi initial oxygen pressures. Sheet polypropylene (PP) and high-density polyethylene (HDPE) as well as injection molded HDPE, HDPP, and LDPE dogbones were exposed at 70°C and 30 psi initial oxygen pressure. Immersion of LDPE films was done under and above water at 80°C and 50 psi initial oxygen pressure. Permeation cup tests were done to determine the permeability of water vapor across the LDPE film at 60, 70, and 90°C. The extent of oxidation and functionality was monitored using ATR-FTIR and was consistent with previous work on polyethylene-oxygen reaction pathways. The permeability of the LDPE films and carbonyl content increased linearly with temperature, oxidation time, and oxygen pressure. The dogbones studied showed little change in mechanical properties.
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    Numerical Modeling of Stress Corrosion Cracking in Polymers
    (2015-12) Ge, Hanxiao
    Polymeric materials have been increasingly used for structural purposes in civil infrastructures. However, stress corrosion cracking has been a critical issue that affects the service lifetime of polymer components. My preliminary study showed that polyethylene may be severely corroded in an oxidizing environment and lose its fracture resistance property. Experimental methods have been primarily adopted to investigate stress corrosion cracking in polymers; however, these approaches are expensive to apply, and may fail to account for certain aspects of this chemo-mechanical process. Therefore, a numerical approach is needed to investigate this issue. A unified chemo-mechanical model is developed to predict the stress corrosion cracking (SCC) of a viscoplastic polymer. This model is applied to the specific case of high density polyethylene (HDPE) exposed to a chlorinated environment at a constant stress load. This chemo-mechanical model is comprised of three components, each capturing a critical aspect of SCC. An elastic-viscoplastic constitutive model is adopted to predict the time-dependent creep behavior of HDPE, and the model parameters have been calibrated through tensile testing. This constitutive model has been implemented in finite element analysis by using a user-defined material subroutine. The polymer fracture property is considered to be dependent on the extent of corrosion, and this dependence is implemented with a cohesive zone model. A chemical kinetics and diffusion model is utilized to predict the degradation of fracture properties in the material as a result of reactions and migration of chemical substances. The coupled chemo-mechanical simulation is accomplished by integrating the chemical reaction calculation into finite element analysis via user defined subroutines. Two modes are considered for failure of the polymer: excessive plastic deformation or catastrophic unstable crack growth. At high stresses, the failure is primarily due to excessive plastic deformation. At low stresses, chemical reactions and diffusion are the dominant factors leading to failure. In addition, two distinct patterns of crack growth (reaction-driven or diffusion-driven) are revealed at various disinfectant concentrations at low stress levels. In reaction-driven crack growth, material degradation is localized at the crack tip, and crack growth rate is a constant throughout the simulated lifetime. However, when diffusion dominates, the entire specimen ligament may be severely degraded, and crack growth accelerates at the end of component lifetime. The current simulation framework allows exploring the interaction of various factors in stress corrosion cracking, such as disinfectant concentration, loading, and temperature. The framework is also general enough to be implemented for other polymeric materials and corresponding corrosion mechanisms. In the future, the proposed chemo-mechanical modeling approach may be expanded to analyze the performance of a variety of materials under stress corrosion cracking. In addition, a stochastic methodology may be incorporated to account for the variances in loading, as well as material properties.
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    On the Intrinsic Kinetics of Polyethylene Pyrolysis
    (2023-05) Mastalski, Isaac
    Global plastic use has grown exponentially over the past several decades, and this has led to a concomitant increase in plastic waste. Because current plastics, and polyolefins such as polyethylene in particular, have become a necessity for modern life, it is unlikely that more sustainable, alternative plastics can displace them anytime soon, so one of the best ways to mitigate plastic waste is to develop more sustainable, alternative recycling methods. Pyrolysis, or thermal degradation under an inert atmosphere, shows great promise in this regard, since it is capable of chemically recycling plastics back to their constituent monomers or to value-added chemicals. However, knowledge of the mechanisms and reaction kinetics underlying polyethylene pyrolysis remains extremely lacking, hindering development of large-scale plastic recycling capabilities. Therefore, the primary objective of this thesis was to investigate those kinetics and shed new light on the reasons behind the vast discrepancies reported in the literature. Fundamental understanding of polyethylene pyrolysis has previously been limited due to an inability to obtain intrinsic reaction kinetics; instead, the literature presently reports only apparent kinetics, which are a combination of intrinsic kinetics and a variety of other transport and system design limitations. In this thesis, an extensive summary of these limitations in other works is presented, and a new system, known as the Pulse-Heated Analysis of Solid Reactions, or PHASR, system was developed to overcome these limitations. The PHASR system is uniquely capable of operating under “isothermal, reaction-controlled” conditions, at which intrinsic kinetics can reliably be measured. The PHASR system was validated extensively to ensure operation in this desired regime, and detailed descriptions of the reactor setup and experimental methodologies are presented. Alongside this system, a second, Visual PHASR system was developed as well, to enable visualization of polyethylene pyrolysis reaction phenomena for the first time, via integrated high-speed photographic equipment. The method of PHASR was then used to study the intrinsic kinetics of polyethylene pyrolysis. Conversion of low-density polyethylene to pyrolysis products was measured over a range of reaction temperatures (550 to 650 °C) and reaction durations (20 ms to 2.0 s), and three distinct product lumps were characterized via integrated gas chromatography and a microgram-resolution balance. Lumped intrinsic reaction kinetics were calculated using these product fractions. The results were further validated by applying a generalized Rice-Herzfeld radical reaction model to the polyethylene pyrolysis system; good agreement was found between this first principles approach and the PHASR experimental data. Additionally, extensive characterization was performed on the residues left behind in PHASR post-pyrolysis, and this helped elucidate new insights into the different reaction timescale regimes that are present during polyethylene pyrolysis.
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    Supporting data for Crosslinked polyolefins through tandem ROMP/hydrogenation
    (2024-03-14) Hillmyer, Marc A; Sample, Caitlin S; Hoehn, Brenden D; hillmyer@umn.edu; Hillmyer, Marc A; Hillmyer Research Group
    These files contain primary data along with associated output from instrumentation supporting all results reported in Sample et al. "Crosslinked Polyolefins Through Tandem ROMP/Hydrogenation". Crosslinked polyolefins have important advantages over their thermoplastic analogues, particularly improved impact strength and abrasion resistance, as well as increased chemical and thermal stability; however, most strategies for their production involve post-polymerization crosslinking of polyolefin chains. Here, a tandem ring-opening metathesis polymerization (ROMP)/hydrogenation approach is presented. Cyclooctene (COE)-co-dicyclopentadiene (DCPD) networks are first synthesized using ROMP, after which the dispersed Ru metathesis catalyst is activated for hydrogenation through addition of hydrogen gas. The reaction temperature for hydrogenation must be sufficiently high to allow mobility within the system, as dictated by thermal transitions (i.e., glass and melting transitions) of the polymeric matrix. COE-rich materials exhibit branched-polyethylene-like crystallinity (25% crystallinity) and melting points (Tm = 107 °C), as well as excellent ductility (>750 % extension), while majority DCPD materials are glassy (Tg = 84 °C) and much stiffer (E = 710 MPa); all materials exhibit high tensile toughness. Importantly, hydrogenation of olefins in these crosslinked materials leads to notable improvements in oxidative stability, as saturated networks do not experience the same substantial degradation of mechanical performance as their unsaturated counterparts upon prolonged exposure to air at high temperature.
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    Supporting Data for Tandem ROMP/Hydrogenation Approach to Hydroxy-Telechelic Linear Polyethylene
    (2022-04-11) Sample, Caitlin S; Kellstedt, Elizabeth A; Hillmyer, Marc A; hillmyer@umn.edu; Hillmyer, Marc A; Hillmyer Group
    These files contain data along with associated output from instrumentation supporting all results reported in Sample et. al. "Tandem ROMP/Hydrogenation Approach to Hydroxy-Telechelic Linear Polyethylene." In Sample et. al. we found: Hydroxy-telechelic polycycloalkenamers have long been synthesized using ring-opening metathesis polymerization (ROMP) in the presence of an acyclic olefin chain-transfer agent (CTA); however, this route typically requires protected diols in the CTA due to the challenge of alcohol-mediated degradation of ruthenium metathesis catalysts that can not only deactivate the catalysts but also compromise the CTA. We demonstrate the synthesis and implementation of a new hydroxyl-containing CTA in which extended methylene spacers isolate the olefin and alcohol moieties to mitigate decomposition pathways. This CTA enabled the direct ROMP synthesis of hydroxy-telechelic polycyclooctene with controlled chain lengths dictated by the initial ratio of monomer to CTA. The elimination of protection/deprotection steps resulted in improved atom economy. Subsequent hydrogenation of the backbone olefins was performed by a one-pot, catalytic approach employing the same ruthenium alkylidene catalyst used for the initial ROMP. The resultant approach is a stream-lined, atom-economic, and low-waste route to hydroxy-telechelic linear polyethylene that uses a green solvent, succeeds with miniscule quantities of catalyst (0.005 mol%), and requires no additional purification steps.
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    Synergistic Effects of Multiple Aging Stressors on HDPE and HDPP
    (2020-12) Finke, Adam
    The service life for power cables in our nation’s nuclear power plants have been extended beyond 40 years, however no reliable test has been found for determining the remaining lifetime of a cable in service. With experimental data and simulated models, a test and material lifetime prediction are being developed based on changes in electrical, mechanical, and chemical properties as a function of service conditions, temperature, aqueous exposure, material, and time. While it may be next to impossible to test every condition or environment for a material lifetime study, analytical models based on accelerated aging can attempt to predict both tested and untested conditions. By using models obtained from experimental data produced during accelerated aging studies, changes in mechanical properties, chemical properties, and even visual properties can be combined to better understand and more accurately predict aging.The first step to understanding insulation degradation is to understand how environmental conditions like water, temperature, and mineral or an aqueous environment affects polymer aging and how that might accelerate or decelerate aging. The aim of this thesis is to cover what effects combining these conditions might have on polyethylene and polypropylene tensile samples at a moderately elevated temperature. Additionally, how might cycling of conditions age or degrade these samples differently from samples continuously submerged. Tensile properties, hardness measurements, and surface chemical characterization of carbonyl formation through the attenuated total reflection were measured and calculated to determine the synergistic effects of aging polyethylene and polypropylene in distilled water, a copper sulfate solution, and Harrison’s solution at 90ºC through common mechanical properties and aging indicators. Additional studies were started aging both materials at elevated temperatures and oxygen partial pressures, and submersion in water. Results suggest that at 90ºC these mixed conditions did not accelerate aging over dry aged samples within a 16-week period and more time or a greater temperature would be required to create a greater difference between conditions.