AKD (Alkyl Ketene Dimer) size was used to improve the water-resistance of non-asbestos sealing materials.The effects on water resistance of the materials were evaluated for two different AKD addition modes,i.e,through internal addition and through clay modification.The results show that AKD is more effective with internal addition in the cellulose containing furnish than in the aramid fiber-containing one.Compared to traditional papermaking process,the sizing efficiency in sealing materials is much lower,mainly because the fine filler content in the latter is much higher.Through clay modification,the AKD sizing efficiency is markedly improved.