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Effect of photochemical and mechanical degradation mechanisms on polyethylene
Dissertation   Open access

Effect of photochemical and mechanical degradation mechanisms on polyethylene

Siavash Vahidi
Doctor of Philosophy (Ph.D.), Drexel University
Oct 2019
DOI:
https://doi.org/10.17918/kz5q-dz95
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Abstract

Photochemistry--Research Civil Engineering Mechanics Polymer Chemistry
As polyethylene (PE) products become more prevalent in a wide variety of engineering applications, it is critical to understand their chemical and mechanical degradation to prevent premature failure. Sunlight radiation has been recognized as the most severe environmental factor causing chemical degradation in PE. Furthermore, PE is also susceptible to mechanical failure through creep rupture and stress cracking. This dissertation focuses on the impact of these degradation mechanisms on the lifetime of PE. PE products designed for sunlight exposure are required to be stabilized with antioxidant (AO) to retard polymer degradation. In this dissertation, photochemical depletion of AO in PE samples exposed to artificial sunlight was monitored by the oxidative induction time (OIT) testing. A mathematical model was developed by implementing the photochemical reaction scheme, radiation attenuation, and diffusion of AO and oxygen. The model successfully described the AO depletion profiles and obtained a relationship between radiation intensity and AO depletion rate, based on which an equation was proposed to predict the AO depletion in PE in exposure to natural sunlight. Mechanical degradation of PE was studied by investigating creep rupture and stress cracking in thin-wall PE tubes. A test setup was designed to apply static and cyclic internal pressures to thin-wall PE tubes and to record their failure times. The pressure tests were accelerated using elevated temperatures. The failure time (and number of cycles) was predicted using two shifting methods for the dry-cooling applications. Furthermore, this dissertation investigated wave formation of PE geomembranes undergoing thermal expansion during installation. Finite element method (FEM) was used to simulate the formation of waves and quantify the tensile strains along the deformed geomembrane. The model was validated by previous experimental studies. Additionally, the effect of overburden pressure on waves was studied.

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