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Title: The recovery of orientation and the relaxation of residual stresses in calendered acrylic polymers
Author: McDonagh-Smith, A.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2001
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The process of extrusion and calendering involves complex deformations over a broad range of temperatures, which induce various residual stresses and preferred orientations. The primary aim of this project was to study the consequence of calendering conditions, composition and place-to-place variability on aging effects, levels of residual stresses and orientations. Standardised procedures have been developed in this study to monitor the recovery of plastic sheet produced in this way, and to characterise the residual stresses and orientations induced by the processing. In the main technique that was developed, samples were taken at regular intervals across sheets of two copolymer grades and subjected to a series of heat treatments at successively higher temperatures. Other techniques that were used included monitoring dimensional changes following the machining of thin samples from the calendered sheet. The copolymer series consisted mainly of poly (methyl methacrylate) (PMMA) with the addition of 3 or 10% ethyl acrylate (EA). Strong evidence has been found for the existence of three distinct regimes in the recovery of these PMMA/ EA copolymers. It has been argued that recovery prior to the onset of the glass transition is due to aging effects induced particularly by cooler calendering rolls. Control samples have shown that residual thermal and viscoelastic stresses relax between the onset of the glass transition and Tg, whereas orientation relaxes above Tg. The length and curvature changes of samples from the four acrylic sheets were interpreted in terms of these three recovery mechanisms. It was found that residual stresses are highest in the copolymers calendered under conditions of low roll temperatures and high line speeds. The opposite processing conditions, i.e. high roll temperatures and low line speeds lead to increased chain extensions. When the two copolymers were processed with the same roll temperatures and line speed settings, greater chain extensions were induced in the 3% EA copolymer, although the residual thermal stresses were similar. The extension of the chain constitutes one form of orientation which is not directly related to the alignment of segments between entanglements, which was relatively unaffected by calendering conditions or the addition of EA.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available