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Title Decrosslinking of Crosslinked Polyethylene via Ultrasonically Aided Extrusion
URL
Publication Date
Degree PhD
Discipline/Department Polymer Engineering
Degree Level doctoral
University/Publisher University of Akron
Abstract The recycling of waste crosslinked polyethylene (XPE) is an important environmental challenge. This research is aimed at systematically studying the decrosslinking of XPE via ultrasonically aided extrusion by using both experimental and theoretical approaches. Peroxide crosslinked high and low density polyethylene (XHDPE and XLDPE) were both used as the model XPE. Ultrasonic single- and twin-screw extruders (SSE and TSE) were used to carry out decrosslinking of XPE at various processing conditions. Two screw designs were used for TSE. The first one, a compounding screw design, contained kneading and forward conveying elements. The second, a decrosslinking screw design, contained forward and reverse conveying elements. Processing characteristics, gel fraction, crosslink density, rheological, thermal, tensile properties and morphology of XPE and decrosslinked XPE were investigated. It was found that the imposition of ultrasound during extrusion of XPE improved processability by reducing the pressure. The mechanical shearing caused by screw rotation in SSE and TSE led to a mechanical decrosslinking of XPE, showing a significant decrease of the gel fraction and crosslink density. More intensive mechanical decrosslinking of XPE occurred in SSE and TSE using decrosslinking screws due to a higher barrel pressure in those setups suppressing wall slip. The imposition of ultrasound further decreased the gel fraction and crosslink density of XPE due to ultrasonic bubble cavitation leading to network rupture. The largest reduction of gel fraction and crosslink density achieved was 81% and 98%, respectively. Decrosslinking occurred to a greater extent in XHDPEs of higher gel fraction and crosslink density. It was determined that the type of bond breakage during decrosslinking of XHDPE, i.e. crosslinks or main chains, was affected by the degree of crosslinking, indicating that bond energy was not the only structural parameter influencing the type of bond breakage. The activation energy for flow of sol of decrosslinked XHDPE increased with a preferential breakage of main chains during decrosslinking of XHDPE leading to a generation of chain branches. The presence of long-chain branching in XLDPE caused a preferential breakage of main chains during decrosslinking of XLDPE. The dynamic properties of decrosslinked XHDPE and XLDPE showed behaviors of critical/post-critical gel due to the presence of a lightly-crosslinked network. After decrosslinking of XHDPE of an intermediate degree of crosslinking, the sample exhibited higher crystallinity and melting temperature and better tensile properties due to the preferential breakage of crosslinks. A 48% increase of Young’s modulus, a 28% increase of yield stress, a 7.5 % increase of stress at break and a 28% increase of strain at break relative to the original XHDPE were achieved.A process model describing ultrasonic decrosslinking of XHDPE was proposed. The proposed model was able to calculate the distribution of temperature, pressure, velocity, gel fraction and crosslink density of…
Subjects/Keywords Polymers; Plastics; Crosslinked Polyethylene, Decrosslinking, Extrusion, Ultrasound, Simulation
Contributors Kyu, Thein (Committee Chair); Isayev, Avraam (Advisor)
Language en
Rights unrestricted ; This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.
Country of Publication us
Format application/pdf
Record ID oai:etd.ohiolink.edu:akron1428160856
Repository ohiolink
Date Indexed 2016-12-22
Grantor University of Akron

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