How to remove Kapton polyimide film from Pyralux cupper sheet?
A Dupont rep replied back with: 


  Attached a method to etch the Kapton but the problem will be to remove
  the Pyralux LF adhesive which holds the copper onto the Kapton. There
  isn't a good way to remove this adhesive.


Also, as I cannot attach PDF documents here - I am adding a screenshot of "Faster Caustic Eteching of Kapton H, E, and KN Types": 


In addition, I found a thesis written by `Gordon Michael Coutts" where he modifies the above formula as follows: 


  bulk material is etched from the Kapton substrate using 9 wt. % KOH in
  a solution of 80 wt. % ethanol and 20 wt. % water. The static etch
  time is 15 minutes to remove 3.5mil of Kapton as shown in Figure
  4.18e. Upon completion of etching, the substrate must be rinsed under flowing DI water for at least 5 minutes to remove insoluble reacted
  material [85]. The substrates must then be baked for 30 minutes at
  100ºC to remove moisture absorbed during the etch process [page 102 or 120/203]


to sum things up, DuPont's paper states that 0.4mil (6mils over 15 minutes) of Kapton is etched per minute when using 6% KOH with 80/20 ethanol/water ratios @ 70C. While David's formula etches 3.5mils of Kapton in 15 minutes when using 9% KOH instead of 6%.