Process for producing improved rigid polyisocyanurate foams based on aromatic polyester polyols and ethylene oxide-based polyether polyols

The present invention relates to a process for producing rigid polyisocyanurate foams, wherein (A) polyisocyanates are mixed with (B) compounds having isocyanate-reactive hydrogen atoms, (C) flame retardants, (D) blowing agents, (E) catalyst at an isocyanate index of at least 220 to afford a reaction mixture and cured to afford the rigid polyisocyanurate foam, wherein the component (B) comprises at least one aromatic polyester polyol (b1) and at least one polyether polyol (b2), the polyester polyol (b1) has an average total functionality of ≤ 3.0 and ≥ 1.7 and the polyether polyol (b2) has a hydroxyl number of 160-350 mg KOH/g and is produced by alkoxylation of a starter or starter mixture having an average total functionality ≤ 3.5 and ≥ 1.5, wherein as the alkylene oxide for producing polyether polyol (b2) at least 80% by weight of ethylene oxide is employed and comprises at least 90% primary hydroxyl end groups and wherein the mass ratio of the component (b1) to component (b2) is ≤ 3 and ≥ 1 and the blowin