| abstract |
A method for preparing rigid polyurethane (PUR) foams or rigid polyisocyanurate (PIR) foams in which method the rigid PUR or PIR foam is prepared by reacting a composition (C) comprising: at least one isocyanate-reactive component (B1) having functional groups selected from hydroxyl, amine and thiol groups; at least one isocyanate component (A1) having an average functionality of less than 2.70; and at least one blowing agent [blowing agent (BA), herein after]; with the proviso that the overall average functionality [Fn,avg(A), herein after] of all isocyanate components present in the composition (C) is less than 2.70; wherein the composition (C) is characterized by an isocyanate index X, wherein the rigid PUR or PIR foams are produced by depositing the composition (C) between two gas-tight facing sheets and wherein the rigid PUR or PIR foam is characterized by a difference Δλ between the initial thermal conductivity value λini and the aged thermal conductivity value λaged of said rigid PUR or PIR foam wherein: when X ≤ 200 then Δλ < 1.35; and when X > 200 then Δλ < [6.49 - (4.46*F n,avg(A)) - (0.02348*X) + (0.492*Fn,avg(A)*Fn,avg(A)) + (0.01343*Fn,avg(A)*X) + 0.3]. |