Process for making a flexible polyurethane foam having a hardness gradient

A molded flexible polyurethane comprising foam with a hardness gradient, wherein said foam comprises at least:

• a top layer which has a thickness which corresponds to around 25% of the total thickness of the foam,
• a bottom layer which has a thickness which corresponds to around 25% of the total thickness of the foam,
• a foam elasticity Ef in the bottom layer of said foam which is at least 3 times higher than in the top layer of said foam, wherein said foam elasticity for each layer is obtained using a single degree of freedom mass-spring-damper system consisting of a fixed mass and a sample of the foam layer, wherein the foam elasticity for each layer is represented by formula [2]:
• E
• f
• =
• ω
• n
• 2
• ⁢
• m
• ⁢
• h
• A
• [
• 2
• ]
• with
• ωn=a natural frequency of the foam layer
• m=the fixed mass
• h=a thickness of the foam layer
• A=a cross-section area of the foam layer
• and wherein said foam is made using a reactive foam formulation, said reactive formulation formed by mixing at an isocyanate index in the range 70-130:
• an isocyanate-reactive composition (a) comprising
• a polyether polyol (a1) having an oxypropylene (PO) content of 51-100% by weight, an oxyethylene (EO) content of 0-49% by weight, an average nominal hydroxyl functionality of 2-4 and an average molecular weight of 2000-7000, and
• optionally a polyether polyol (a2) having an oxyethylene content of 50-95% by weight, calculated on the weight of this polyol wherein the weight ratio of polyol (a2) in the isocyanate-reactive composition (a) is in the range of 0 to 20% by weight, calculated on the total weight of the isocyanate-reactive composition (a), and
• a polyisocyanate composition (b) having an NCO value in the range of 21 up to 25.5% and comprising 0-12% by weight methylene diphenyl 2,4′-diisocyanate (2,4′-MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition and a balance of polymeric MDI and methylene diphenyl 4,4′-diisocyanate (4,4′-MDI).