The grafting modification method may be any known method, such as the method of reacting in a melted state disclosed in Japanese Patent KOKOKU No. 43-27421, the method of reacting in a solution state disclosed in Japanese Patent KOKOKU No. 44-15422, the method of reacting in a slurry state disclosed in Japanese Patent KOKOKU No. 43-18144 and the method of reacting in a vapor state disclosed in Jap