Softening Point of 2-Liter PET Soda Bottles

When exothermic demonstrations are performed using 2 L polyethylene terephthalate (PET) soda bottles, the bottles sometimes soften and shrink.  A great example of an exothermic reaction that can do this to PET bottles is the catalytic decomposition of 30% hydrogen peroxide.  Wikipedia has mentioned that the glass transition of PET is about 70 C, so we set out to check to see if that corresponded to the softening point of the PET bottles.  We heated a large kettle of water to various temperatures with a stove, checking the water temperature with a Vernier temperature probe.  At selected temperatures, we placed a 2 L PET bottle nearly completely into the water (with its opening above water and the interior NOT flooded with water) for one minute.  The results are shown in the picture below.  At 65 C the bottle is unchanged, but at roughly 70 C and above, the bottle deforms.  The extent of the bottle deformation seems to increase as the temperature increases. Thanks to Kristine Campbell for assistance with the experiment.