Heat-expandable microspheres and application thereof

[0137]An aqueous dispersion medium was prepared by adding 150 g of sodium chloride, 50 g of colloidal silica containing 20 wt % of silica, 4.0 g of polyvinyl pyrolidone and 1.0 g of ethylenediaminetetraaceticacid tetrasodiumsalt to 600 g of deionized water and adjusting the pH of the mixture within the range from 2.0 to 3.0.

[0138]An oily mixture was prepared by mixing 76 g of acrylonitrile, 40 g of methacrylonitrile, 180 g of methacrylic acid, 3 g of 2-hydroxyethylacrylate, 1 g of 1,9-nonanediol diacrylate, 30 g of isopentane, 30 g of isooctane and 8 g of the liquid containing 50% of di-sec-butyl peroxydicarbonate.

[0139]The aqueous dispersion medium and the oily mixture were mixed and agitated with a Homo-mixer to be prepared into a suspension. Then the suspension was transferred to a compressive reactor of 1.5-liter capacity, purged with nitrogen, and polymerized at 60° C. for 20 hours with agitation under the initial reaction pressure of 0.2 MPa. The resultant polymerization product