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Experiment 12: BoPET


2 March 2010 by Quest 2, to link in with Chapter 12 of our book.


To investigate the properties of biaxially-oriented polyethylene terephthalate polyester film, or boPET polyester film (commonly known by the brand name Mylar).


Take a sheet of mylar and try pulling it apart. While pulling on the sides, poke the middle with a pen or other sharp object. Tear the sheet from the edge.

Materials & Equipment

  • Mylar sheet (the wrapper from a mailed magazine).
  • Pen.


When James first pulled the edges of the mylar it held strongly until suddenly it split apart with a bang. This was a great source of amusement to those watching.

The mylar was almost impossible to pull apart unless it had a small hole or tear, in which case it had next to no strength at all.


  • Mylar is very strong if it is intact, but very weak if it is not.
  • Suddenly breaking mylar makes kids go giggly.