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Title: Effects of high pH solutions on polyethylene based materials
Author: Jones, A. J.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2005
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High Density Polyethylene (HDPE) and HDPE filled with Carbon Black (CB) and/or Electro Graphite (EG) are utilized in commercial applications, whereby the materials come into contact with highly alkaline solutions and so the effects of such environments are critical. This study investigates the potential environmental stress cracking (ESC) effect of pH 13.5 Sodium Hydroxide (NaOH) solution on these polyethylene based materials. A range of temperatures from 23°C to 80°C was used. Various mechanical testing methods were employed, including constant strain three point bend, tensile creep, constant strain rate tensile tests and three point bend creep tests. The three point bend techniques revealed the clearest differences between the tests performed in NaOH and air. Samples were subsequently investigated using optimal microscopy, scanning electron microscopy (SEM) and IR spectroscopy techniques. PE was found to undergo ESC in the presence of NaOH at combinations of high strains (≥14%) and high temperatures (≥60°C). The addition of carbon filler reduced the ductility of the material and increased its strength. ESC effects were observed at lower strains for the filled materials and this was related to the amount of added filler. Addition of filler was found to embrittle the material, whereby CB had a more significant effect than the equivalent amount of EG. The CB particles were finer, resulting in the polymer chains being less easily deformed. The EG particles were much larger than the CB ones and due to the nature of graphite, a lubrication effect on a microscopy level, resulted in the samples reaching greater strains than the CB filled ones. The FTIR study did not reveal any chemical changes in the damaged samples; however, the technique used was limited in its effectiveness. When samples tested in both air and NaOH became damaged under the same test conditions, the material properties defined the type of damage occurring. The SEM study did clearly show an embrittling effect resulting from the NaOH, when damage occurred in NaOH but not in air; the fracture surfaces were seen to be smoother for the NaOH tests, when compared to the control tests. The proposed action of the NaOH is to cause embrittlement of the polymer by causing surface energy reductions and hydrolysis, leading to the breakdown of the polymer backbone.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available