Preparation method of improved polyisocyanurate rigid foam plastic based on aromatic polyester polyol and oxirane-based polyether polyol

The invention relates to a method for producing a rigid polyisocyanurate foam, in which (A) a polyisocyanate is mixed with (B) a compound having hydrogen atoms reactive to isocyanate groups, (C) a flame retardant, (D) a blowing agent, (E) a catalyst to form a reaction mixture and cured to form the rigid polyisocyanurate foam when the isocyanate index is at least 220, component (B) comprises at least one aromatic polyester polyol (b1) having an average functional group of < = 3.0 and > = 1.7 and at least one polyether polyol (b2) having a hydroxyl number of 160 mg KOH/g to 350 mg KOH/g and being produced by alkoxylation of a starter or starter mixture having an average functional group of < = 3.5 and > = 1.5, the component (B) comprising at least one aromatic polyester polyol (b1) having an average functional group of < = 3.0 and > = 1.7 and at least one polyether polyol (b2) having a hydroxyl number of from 160 mg KOH/g to 350 mg KOH/g, the alkylene oxide groups for producing the polyether polyol (b2) are at