To provide a building panel formed to have a high fire retarding function, and even both of light weight and a high heat insulating function by using an expanded material capable of showing a fire retarding function equivalent to or above the fire retarding level 2, and specifying the density of the expanded material so as to become equal to or smaller than the prescribed value.
Polyhydroxy compound and a polyisocyanate compound are made to react with each other in the presence of a foaming agent, a catalyst and a foam shaping agent and other assistants containing a fluororesin compound, thereby preparing isosyanurate modified special and highly humid polyurethane foam having density equal to or lower than 0.04g/cm2. As the hydroxy compound, the use of iso(metha)phthalic acid, tere(para)phthalic acid or the like is effective for showing a high fire retarding function. In addition, hydrofluorocarbon or the like designated as the third generation of Freon is available as the foaming agent. Also, for efficiently attaining low density. HCFC' s are used not alone but together with HFC's. Alternatively, the use of the HFC's alone is more effective.
HIRAIWA TOSHIHIKO
ISHII MASASHI
