Examples of the radical polymerization initiator include persulfates such as potassium persulfate and ammonium persulfate; water-soluble initiators such as aqueous hydrogen peroxide, t-butylhydroperoxide, t-butylperoxymaleic acid, succinic acid peroxide, and 2,2'-azobis-(2-N-benzylamidino)propane hydrochloride; oil-soluble initiators such as benzoyl peroxide, cumene hydroperoxide, dibutyl peroxide