From trying to find thermochromic materials, this is some data I got along
the way, from trying Mylar film (or metalised plastic anyway) ;
bubbles/wrinkles 75 deg C, shrinks 80 degree C and shrunk to 1/4 95 deg C so
pvc and polythene probably similar


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/