Abstract: Hydrophobic precipitated SiO₂ was prepared by in-situ modification with hydroxyl silicone oil as modifier.FT-IR  analysis indicated that hydroxyl silicone oil had been grafted onto SiO₂ surface by chemical bonds.SEM indicated that the agglomerate phenomenon and dispersion property were improved by in-situ modification,and the compatibility with organic matrix,such as polypropylene resin,was increased.Hydrophobic property test showed that when the dosage of hydroxyl silicone oil 203-B and 203-D both reached 17.9% (mass ratio of hydroxyl silicone oil and SiO₂),the hydrophobic degree increased to 24.59% and 22.17% respectively;the smaller relative molecular weight of hydroxyl silicone oil,the higher hydroxyl content,the more obvious modification effect. In addition,in-situ modification also reduced the oil-absorb ratio of precipitated SiO₂.

Key words: precipitated silica, hydroxyl silicone oil, in-situ modification, hydrophobic degree, compatibility