Video/animation icon  Recovery of molecular orientation in a blow-moulded PET bottle

 
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Description: PET for bottles is processed in an amorphous state. During the stage in the process where the bottle is formed, the amorphous polymer molecules are stretched out and then 'frozen' in their new, more aligned form by cooling below the glass transition temperature. Reheating above that temperature (about 75 °C) allows them to re-orient back towards their original random arrangement and the bottle shrinks in length and diameter. A small proportion of the molecules also get trapped in a special form of crystal known as a 'fringed micelle'. Crystalline PET melts at over 250 °C so recovery of the frozen-in orientation on gentle heating is never 100%. PET will also crystallise in the more normal spherulitic form but this takes much longer at temperatures above the glass transition temperature.
Keywords: polymers • recycling
Categories: Science approaches
Materials > Polymers & elastomers > Plastics > Thermoplastics > Polyethylene terephthalate (PET)
Processes > Thermal treatments
Testing, analysis & experimentation
Created by: DoITPoMS, University of Cambridge
Published by: DoITPoMS, University of Cambridge
License: This resource is released under the Creative Commons Attribution Non-Commercial Share Alike license (2.0 UK: England & Wales).
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Date created: 05 September 2006
Date added: 02 December 2009
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Resource ID: 2380