The silane coupling agent may be an organosilane compound, preferably one represented by the formula Rn SiX4-n, wherein n is 1, 2 or 3; and X is, for example, a halogen atom such as chlorine atom or bromine atom; an alkoxy group such as methoxy, ethoxy, propoxy or butoxy, an acyloxy group such as acetoxy, or another hydrolyzable functional group.