The solubility by rubbing was measured according to JISK-3304. That is, a test specimen (cross section: 15 mmx20 mm) with a fixed weight was placed on the surface of a film wetted by tap water that had been adjusted to 40° C. and dissolved by rubbing for 10 minutes by rotating the film.from the weights before and after dissolution by rubbing, the solubility by ubbing per fixed area was determined by the following equation.