Oil-in-water type emulsion and method for producing oil-in-water type emulsion

A method for producing an oil-in-water emulsion, comprising the steps of:

• a) preparing a fumed silica at a secondary aggregate level by mixing a hydrophobic-rich fumed silica raw material in which surface silanol groups have been subjected to a hydrophobic treatment, with a hydrophilic-rich fumed silica raw material in which surface silanol groups remain, to form a composite mixture, the composite mixture having a ratio of all hydrophobic groups to all hydrophilic groups of from 80:20 to 20:80;
• b) producing an aqueous dispersion by adding the composite mixture obtained from step a) to water and shearing at a shearing speed which allows rearrangement of hydrophilic and hydrophobic lower order aggregates, the resulting aqueous dispersion containing fumed silica particles in which the lower order aggregates are aggregated with each other to form higher order aggregates aggregated by non-chemical bonds, thereby forming a network structure surrounding a hydrophobic space inside the network structure;
• and
• c) forming an oil-in-water emulsion by adding an oil to the aqueous dispersion produced in step b),
• wherein the oil-in-water emulsion comprises composite particles in which the oil is inside the network structure formed by the fumed silica particles at a secondary aggregate level, and
• wherein the oil comprises a water and oil-repellant fluorinated polyorganosiloxane of an average composition:
• R1aR2bSiO(4-a-b)/2 (1)
• wherein the formula (1):
• where R1 are the same or different from each other, and are substituted or unsubstituted, saturated or unsaturated monovalent hydrocarbon groups having 1 to 25 carbon atoms, substituted or unsubstituted aromatic groups having 6 to 30 carbon atoms, a hydroxyl group, alkoxy groups having 1 to 6 carbon atoms, or a hydrogen atom;
• R2 are the same or different from each other, and are hydrocarbon groups which are saturated or unsaturated monovalent hydrocarbon groups having up to 25 carbon atoms, or aromatic groups having 6 to 30 carbon atoms, and in which hydrogen atoms bonded to carbon atom have been substituted with fluorine;
• b is a positive number which does not include 0;
• and a+b is 0.3 or more and less than 2.5.