Adherence. Normal PLA/ABS or other plastic filaments will shrink when they cool down, and cause the thin layer of material to pull and separate from the copper. TPU is rubber-like and will have less inner stress to pull the material off. Also PLA absorbs humidity and will tend to change volume in a solution, with the same propensity to separate from the copper during the etching. TPU has more impermeability.
I have also tried using adhesion glues and hair sprays. Sometimes a thin application of hair spray (removed with a quick spray of isopropyl before the etching) helps a bit to define the edge traces. However I found that cleaning the copper very well, having a clean chamber in the printer and controlling well the etching temperature to reduce the etching time has the same effect and is less burdensome.
Good question. Not yet. When I carried out the tests (3+ years ago) I didn't have PETG filament around. Then TPU worked well for me and I have been sticking with it. I have a couple of PETG spools now so I will certainly try it next time!
I have also tried using adhesion glues and hair sprays. Sometimes a thin application of hair spray (removed with a quick spray of isopropyl before the etching) helps a bit to define the edge traces. However I found that cleaning the copper very well, having a clean chamber in the printer and controlling well the etching temperature to reduce the etching time has the same effect and is less burdensome.