. . . "Further although the kind and amount of remaining functional groups such as amino, imino and methylol groups are not limited specifically insofar as the resin fulfils the above requirements as to the solvent dilution ratio and molecular weight, the preferred amount of imino groups (inclusive of amino groups), as well as methylol groups, is 0.2 to 2.0 moles, more preferably 0.5 to 1.5 moles, per tr" .