THF-insoluble matter (%)=[{W 1???(W 3 +W 2)}/(W 1 ???W 3)]???100. [0174] Incidentally, to measure the THF-insoluble matter of samples not containing components other than the resin, such as the binder resin, the resin weighed in a stated quantity (W1 g) is put to the same steps as the above to determine THF-soluble matter (W2 g).