0 1 n given from the top using a thermal mechanical analyzer tma tma q400 product of ta instruments the displacement value of the measuring terminal in the height direction was measured the temperature at which the displacement value turned positive was defined as the foaming starting temperature ts and the temperature at which the displacement value reached the maximum value was defined as the maximum foaming temperature tmax the decomposition temperature tc of the chemical foaming agent was determined as follows a chemical foaming agent 10 mg was charged in an aluminum container having a diameter of 6 mm and a depth of 5 mm the container was heated from 80 u00b0 c to 300 u00b0 c at a heating rate of 2 u00b0 c min with a thermogravimetric differential thermal analysis tg dta equipment tg dta6300 product of seiko instruments inc and a weight loss curve was drawn the inflection point of the curve was defined as the decomposition temperature tc a polymerization reaction vessel was charged with water 250 parts by weight sodium chloride 85 parts by weight as a dispersion stabilizer colloidal silica product of adeka corporation 20 wt 25 parts by weight and polyvinylpyrrolidone product of basf 0 2 parts by weight to prepare an aqueous dispersion medium subsequently to the aqueous dispersion medium was added an oily substance consisting of acrylonitrile an 30 parts by weight methacrylonitrile man 50 parts by weight and methacrylic acid maa 20 parts by weight as polymerizable monomers zinc hydroxide 0 5 parts by weight epoxy resin trade name jer630 product of japan epoxy resin co ltd 0 2 parts by.