Process for making a flexible polyurethane foam having a hardness gradient
A molded flexible polyurethane foam having a hardness gradient going from soft to hard from the top to the bottom of the foam. The hardness gradient in the foam is a result of a foam elasticity gradient which arises from a polymer elasticity gradient and/or density gradient. A method for producing a flexible foam having a hardness gradient and a reactive mixture suitable for making said flexible foam is disclosed. Furthermore, the use of the flexible foams having a hardness gradient in matrasses, cushions for seating (more in particular for use in automotive seating), furniture, automotive under-carpets and dash insulators is disclosed.
1. 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 E f 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
ω n =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).
2. The molded flexible foam according to claim 1 , having a polymer elasticity Ep in the bottom layer of said foam which is at least 2 times higher than in the top layer of said foam and wherein said polymer elasticity for each layer corresponds to formula [1]:
E
p
=
E
f
R
2
[
1
]
wherein
R is the relative density of polyurethane polymer to the foam layer, which is defined as
ρ is the polyurethane polymer density, which equals 1200 kg/m 3 , and
ρ f is the polyurethane foam density of the foam layer, which is measured according to ISO 845.
3. The molded flexible foam according to claim 1 , having a foam density pf in the bottom layer of said foam which is 10% up to 40% higher than in the top layer of said foam.
4. The molded flexible foam according to claim 2 , having a hardness gradient has a foam density ρ f in the bottom layer of said foam which is 10% up to 40% higher than in the top layer of said foam.
5. The molded flexible foam according to claim 2 , having a polymer elasticity E p in the top layer which is lower than the polymer elasticity E p in the middle section of the foam.
6. The molded flexible foam according to claim 1 , having a foam density pf in the bottom half of the foam which is higher than the foam density ρ f in the top half of the foam.
7. The molded flexible foam according to claim 2 , having a polymer elasticity E p in the top layer which is lower than the polymer elasticity E p in the middle section of the foam and a foam density ρ f in the bottom half of the foam which is higher than the foam density ρ f in the top half of the foam.
8. The molded flexible foam according to claim 1 , wherein the polyisocyanate composition (b) comprises at least 40% of the 4,4′-MDI calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition.
9. The molded flexible foam according to claim 1 , wherein the reactive foam formulation further comprises a filled polyether polyol (a3) in an amount up to 30% by weight calculated on the total weight of the isocyanate-reactive composition (a).
10. The molded flexible foam according to claim 1 , wherein the reactive foam formulation further comprises blowing agents, catalysts, chain extenders, fire retardants, fillers, and surfactants.
11. The molded flexible foam according to claim 1 , wherein the reactive foam formulation further comprises a blowing agent, said blowing agent comprising at least water and the amount of water used is 0.5 up to 10% by weight calculated on the total weight of all ingredients being present in the isocyanate-reactive composition (a) used to form the reactive foam formulation.
12. A process for making the molded flexible foam according to claim 1 , said process comprising at least the steps of:
(i) mixing the polyisocyanate composition (b) with the isocyanate-reactive composition (a) at an isocyanate index in the range 70-130 to obtain the reactive foam formulation, and then
(ii) casting the reactive foam formulation obtained in step (i) into a mold to obtain flexible foam having a hardness gradient, and then
(iii) demoulding the obtained flexible foam having a hardness gradient characterized in that step (iii) is performed such that there is a temperature difference (ΔT) of at least 25° C. between the temperature of the reactive foam formulation (T chemicals ) and the temperature of the mold (T mold ).
13. The process according to claim 12 , wherein the temperature difference ΔT between the initial reactive foam formulation used (T chemicals ) and the temperature of the mold (T mold ) is at least 35° C.
14. The process according to claim 12 , wherein the minimum temperature of the initial reactive foam formulation used (T chemicals ) is 10° C.
15. The molded flexible foam according to claim 1 , wherein the polyether polyol (a2) is present in the isocyanate-reactive composition (a) at 3.3-20% by weight, calculated on the total weight of the isocyanate-reactive composition (a).
16. The molded flexible foam according to claim 15 , wherein the reactive foam formulation further comprises a filled polyether polyol (a3) in an amount of 10.6-30% by weight calculated on the total weight of the isocyanate-reactive composition (a).