PROCESS FOR MANUFACTURING A FLEXIBLE POLYURETHANE FOAM THAT HAS A HARDNESS GRADIENT

Abstract

A molded flexible polyurethane foam exhibiting a hardness gradient that ranges from soft to hard from the top to the bottom of the foam. This hardness gradient results from a foam elasticity gradient, which in turn arises from a polymer elasticity and / or density gradient. A method for producing a flexible foam with this hardness gradient and a suitable reactive blend for manufacturing the foam are described. Furthermore, the applications of flexible foams with this hardness gradient in mattresses, seat cushions (particularly for automotive seats), furniture, automotive underlayment, and dashboard insulation are described.

Description

PROCESS FOR MANUFACTURING A FLEXIBLE POLYURETHANE FOAM THAT HAS A HARDNESS GRADIENT FIELD OF INVENTION The present invention relates to a molded flexible polyurethane foam having a hardness gradient that ranges from soft to hard from the top to the bottom of the foam. The hardness gradient in the foam results from a foam elasticity gradient arising from a polymer elasticity and / or density gradient. Surprisingly, we have now found a new method for producing a flexible foam that has a hardness gradient and a reactive mixture suitable for manufacturing the flexible foam. The invention also relates to the use of flexible foams having a hardness gradient in mattresses, seat cushions, and more specifically for use in automotive seats. BACKGROUND OF THE INVENTION Improvements in passenger compartment comfort remain a key need for the global transportation industry. Since their introduction over 40 years ago, flexible molded polyurethane foams have successfully contributed to the comfort provided by all types of transportation seating. The comfort experience is a combination of many different factors, including the trend toward reduced foam density to give passengers a softer feel while maintaining technical performance specifications, such as providing sufficient continuous support, which involves having a much firmer foam at the bottom. The problem is currently solved by creating a multi-layered foam by adding a softer layer of block foam on top of a standard, firmer molded foam seat cushion. This solution is not ideal as an additional processing step and requires two different foam materials. BRIEF DESCRIPTION OF THE INVENTION According to a first aspect, a foam comprising molded flexible polyurethane with a hardness gradient is described. The foam comprises at least - a top layer which has a thickness (height) that corresponds approximately to 25% of the total thickness (height) of the foam, - a lower layer which has a thickness (height) that corresponds approximately to 25% of the total thickness (height) of the foam, - a foam elasticity Et in the lower foam layer that is at least 3 times greater, preferably 3 to 10 times greater than in the upper foam layer and where the foam elasticity corresponds to formula [2]: iviA / a / zuz ι / u iouoj With ωη = the natural frequency, m = a fixed mass, h = the thickness of the foam sample A = the cross-sectional area of ​​the foam sample According to embodiments, the flexible foam molded according to the invention has a polymer elasticity Epen in the lower layer of the foam that is at least 2 times greater, preferably from 2 to 8 times greater in the upper layer of the foam and where the polymer elasticity corresponds to the formula [1]: [1] With - The relative density (R) defined as R = — Pp - The polymeric density of polyurethane (pp) = 1200 kg / m3, - The density of polyurethane (pt) being measured according to ISO 845 molded flexible polyurethane comprising foam with a hardness gradient, where the foam comprises at least: - a top layer which has a thickness (height) that corresponds approximately to 25% of the total thickness (height) of the foam, - a lower layer which has a thickness (height) that corresponds approximately to 25% of the total thickness (height) of the foam, - A foam elasticity Ef in the lower foam layer that is at least 3 times greater, preferably 3 to 10 times greater than in the upper foam layer and where the foam elasticity corresponds to formula [2]: Et = ωη2— [2] With ωη = the natural frequency, m = a fixed mass, h = the thickness of the foam sample A = the cross-sectional area of ​​the foam sample According to embodiments, the flexible molded foam according to the invention has a foam density pf in the lower foam layer which is 10% to 40% greater than in the upper foam layer. According to embodiments, the flexible molded foam according to the invention has a polymeric elasticity EP in the lower layer of the foam that is at least 2 times greater, preferably from 2 to 8 times greater than in the upper layer of the foam and where the elasticity MA / a / ZUZI / UIOUÜJ polymeric corresponds to formula [1] and a hardness gradient has a foam density pt in the lower foam layer which is 10% to 40% greater than in the upper foam layer. According to embodiments, the flexible foam molded according to the invention has a polymeric elasticity EP in the top layer which is less than the polymeric elasticity EP in the core (middle section) of the foam. According to embodiments, the flexible foam molded according to the invention has a foam density pf in the lower half of the foam which is greater than the foam density pt in the upper half of the foam. According to embodiments, the flexible molded foam according to the invention has a polymer elasticity Epen in the top layer that is less than the polymer elasticity Epen in the middle section of the foam and a foam density pt in the lower half of the foam that is greater than the foam density pf in the upper half of the foam. According to a second aspect, a reactive foam formulation for manufacturing the foam according to the invention is described. The formulation is formed by mixing an isocyanate index in the range of 70-130, preferably in the range of 75-110, more preferably in the range of 75-100 at least: a) A reactive isocyanate 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, preferably at most 20% by weight calculated on the total weight of the polyol (a1), an average nominal hydroxyl functionality of 2-4 and an average molecular weight of 2000-7000, - optionally a polyether polyol (a2) having an oxyethylene content of 50-95% by weight, calculated on the weight of this polyol, wherein the ratio of polyol (a2) to the isocyanate reactive composition (a) is in the range of 0 to 20% by weight, preferably in the range of 0 to 10% by weight, more preferably in the range of 0 to 5% by weight calculated on the total weight of the isocyanate reactive composition (a), and - optionally a filled polyether polyol (a3), also called a polymeric polyol, wherein the weight ratio of the polyol (a3) ​​in the isocyanate reactive composition (a) is in the range of 0 to 30% by weight, preferably in the range of 0 to 20% by weight calculated on the total weight of the isocyanate reactive composition (a) b) a polyisocyanate composition (b) comprising having an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to the embodiments, the polyisocyanate composition (b) in the reactive foam formulation for manufacturing the foam according to the invention is first reacted (i.e., prepolymerized) with a polyol, and the amount of polyol that reacted in the polyisocyanate composition (b) is in the range of 0-40% by weight, preferably in the range of 0-30% by weight, more preferably in the range of 0-20% by weight, calculated on the total weight of the polyisocyanate composition (b). According to embodiments, the polyisocyanate composition (b) is a composition comprising 0-12% by weight, preferably 0-10% by weight of methylene diphenyl 2,4'-diisocyanate (2,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition and an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to embodiments, the polyisocyanate composition (b) is a composition comprising 0-12% by weight, preferably 0-10% by weight of 2,4'-methylene diphenyl diisocyanate (2,4 MDI) and comprising at least 40%, preferably at least 50% by weight of 4,4'-diphenylmethane diisocyanate (4,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition and an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to the modalities, the polyisocyanate composition (b) has an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to the modalities, the polyisocyanate composition (b) has an NCO value in the range of 20 to 25.5%, preferably in the range of 23 to 25.5%. According to certain embodiments, the reactive foam formulation for manufacturing the foam according to the invention further comprises additives such as blowing agents, catalysts, chain extenders, and other additives such as flame retardants, fillers, surfactants, ... According to embodiments, the reactive foam formulation for manufacturing the foam according to the invention further comprises blowing agents, the blowing agent comprising at least water and the amount of water used is from 0.5 to 10% by weight, preferably from 1 to 5% by weight calculated on the total weight of all ingredients present in the reactive isocyanate composition (a) used to form the reactive foam formulation according to the invention. According to a third aspect, a process for manufacturing the flexible foam according to the invention is described, the process comprising at least the following steps: i. mixing the polyisocyanate composition (b) with the isocyanate reactive composition (a) to an isocyanate index in the range of 70-130, preferably in the range of 75-110, more preferably in the range of 75-100 to obtain the reactive foam formulation according to any of claims 9 to 12, and then iii. melting the reactive foam formulation obtained in step i. into a mold to obtain a flexible foam having a hardness gradient, and then iii. demolding the obtained flexible foam having a hardness gradient, characterized in that step iii is carried out such that there is a temperature difference (ΔT) of at least 25-30°C between the temperature of the reactive foam formulation (Tchemicals) and the temperature of the mold (Tmold). According to the modalities, the temperature difference ΔT between the initial reactive foam formulation used (Tchemicals) and the mold temperature (Tmold) is at least 25-30°C, more preferably at least 30-50°C, more preferably the temperature difference ΔT is at least in the range of 35-55°C. ML / a / ZUZ l / UI DUOJ According to the modalities, the minimum temperature of the reactive foam formulation used (Tchemicals) is 10-15°C, preferably Tchemicals is around ambient temperature and the temperature of the mold (Tmold) is at least 50°C and less than 100°C, preferably Tmold is in the range of 55°C to 70°C, more preferably in the range of 60°C to 70°C. According to a fourth aspect, the use of the molded flexible foam according to the invention is described as automotive seats, mattresses, furniture, automotive under-carpets and dashboard insulators. The independent and dependent claims set forth particular and preferred features of the invention. The features of the dependent claims may be combined with features of the independent or other dependent claims, as appropriate. The foregoing and other features, characteristics, and advantages of the present invention will become evident from the following detailed description, taken in conjunction with the accompanying figures, which illustrate, by way of example, the principles of the invention. This description is given for illustrative purposes only, without limiting the scope of the invention. DEFINITIONS In the context of the present invention, the following terms have the following meanings: 1) Isocyanate index or NCO index or index: the ratio of NCO groups to isocyanate reactive hydrogen atoms present in a formulation, given as a percentage: [NCO] X 100 [active H atoms] In other words, the NCO index expresses the percentage of isocyanate actually used in the formulation with respect to the amount of isocyanate theoretically required to react with the amount of isocyanate reactive hydrogen used in a formulation. It should be noted that the isocyanate index as used herein is considered from the standpoint of the actual polymerization process that prepares the foamed material involving the isocyanate ingredient and isocyanate-reactive ingredients. Any isocyanate pool consumed in a preliminary step to produce modified polyisocyanates (including isocyanate derivatives referred to in the prior art as prepolymers) or any active hydrogen consumed in a preliminary step (e.g., that reacted with isocyanate to produce modified polyols or polyamines) is not taken into consideration in the calculation of the isocyanate index. 2) The expression isocyanate reactive hydrogen atoms as used herein for the purpose of calculating the isocyanate index refers to the total active hydrogen atoms in the hydroxyl and amine groups present in the reactive compositions; this means that for the purpose of calculating the isocyanate index in the current polymerization process, a hydroxyl group is considered to comprise one reactive hydrogen and a primary amine group is considered to comprise one reactive hydrogen. 3) The expression Reaction system, Reactive foam formulation and Reactive mixture as MA / a / ZUZI / UIOUÜJ, as used herein, refers to a combination of reactive components used to manufacture a polyurethane comprising foam, where the polyisocyanates are usually kept in one or more containers separate from the reactive isocyanate components. 4) The term average nominal hydroxyl functionality (or, briefly, functionality) is used herein to indicate the average numerical functionality (number of hydroxyl groups per molecule) of the polyol or composition on the assumption that this is the average numerical functionality (number of active hydrogen atoms per molecule) of the initiators used in its preparation, although in practice it will often be somewhat lower due to some terminal unsaturation. 5) The word average refers to the numerical average unless otherwise stated. 6) The term gradient, as used herein, refers to a change in the value of a variable (such as temperature, hardness, elasticity, concentration, etc.) with a change in a given variable, and especially per unit distance in a specific direction. The gradient is given here as a number, where the number is the difference between the maximum and minimum observed values ​​for that variable. For example, the foam according to the invention has a foam elasticity gradient Et, meaning that the foam elasticity Ef in the lower foam layer can be 3 to 4 times greater than in the upper foam layer. 7) The reference to an upper and a lower layer in the foam according to the invention refers to a molded foam having a softer upper layer due to its lower elasticity compared to the upper foam layer, and a harder lower layer due to its greater elasticity compared to the lower foam layer. For the interpretation of the hardness gradient in the foam according to the invention, the upper layer of the molded foam corresponds to the layer in contact with the lower part of the mold, which is the initial stage of the foaming process. The upper foam layer has a thickness (height) that corresponds to up to 25% of the total thickness (height) of the foam.The bottom layer of the molded foam is the layer in contact with the top of the mold, and it is this layer that is formed at the end of the foaming process. The bottom layer of the foam also has a thickness (height) that corresponds to up to 25% of the total thickness (height) of the foam. 8) The term ambient temperature refers to temperatures of approximately 20°C, meaning it refers to intervals within the range of 18°C ​​to 25°C. These temperatures will include 18°C, 19°C, 20°C, 21°C, 22°C, 23°C, 24°C, and 25°C. 9) Unless otherwise stated, the weight percent (stated as % by weight or % by weight) of a component in a composition refers to the weight of the component in the total weight of the composition in which it is present and is expressed as a percentage. 10) Unless otherwise stated, parts by weight (pep) of a component in a composition refers to the weight of the component in the total weight of the composition in which it is present and is expressed as pep. 11) Foam density (pt) refers to the density as measured on foam samples by cutting a parallelepiped of foam, weighing it, and measuring its dimensions. Density is the ratio of weight to volume / measurement according to ISO 845 and is expressed in kg / m³. 12) Polymer density as referred to in this invention refers to the density of the polyurethane polymer (pP) and is assumed to be 1200 kg / m3 (Randall, D. and Lee S., eds., (2002) The Polyurethanes Handbook, London: Wiley). 13) Polymer elasticity (Ep) as referred to in this invention can be estimated from the Gibson and Ashby equation (Gibson, LJ & Ashby, MF (1988) Cellular Solids, Pergamon, Oxford) which describes the relationship between foam elasticity, polymer elasticity, and relative elasticity. Ep can be represented as follows in equation [1]: ^=3 nor With - Et = Elasticity of the foam - The relative density (R) defined as Rpp - The polymeric density of polyurethane (pp) = 1200 kg / m3, - The density of polyurethane foam (pf) being measured according to ISO 845 14) The elasticity of the foam (Ef) as referred to in this invention can be obtained from the frequency of a single-degree-of-freedom mass-spring-damper system, called a vibration transmissibility test, consisting of a fixed mass and a spring sample, see Figure 1a. The natural frequency ωη of the system is the frequency at which the transmissibility is at its maximum, see Figure 1b. Ef can be represented as follows in equation [2]: Ε,.ωη2^ [2] With ωη = the natural frequency, m = a fixed mass, h = the thickness of the foam sample A = the cross-sectional area of ​​the foam sample The present invention relates to a flexible polyurethane foam having a hardness gradient that ranges from soft to hard from the top to the bottom of the foam. This hardness gradient results from a foam elasticity gradient arising from a polymer elasticity gradient and / or a density gradient. The resulting hardness gradient creates a foam with a softer top layer and a harder bottom layer, making it ideal for manufacturing more comfortable foam seats, for example, for use in automotive seating. In this way, the foam has a softer surface feel while the bottom layer ensures sufficient support. The invention describes accordingly a molded flexible polyurethane comprising foam which has a hardness gradient, wherein the foam has at least - an upper layer which corresponds to the layer that is in contact with the lowest part of the mold and which is when the foam formation process is taken into account, formed at the beginning (at the start) of the foam formation process and where the upper layer has a thickness (height) that can correspond up to 25% of the total thickness (height) of the foam, - a lower layer which corresponds to the layer that is in contact with the uppermost part of the mold and which is when the foaming process is taken into account, formed at the end of the foaming process and where the lower layer of the foam has a thickness (height) that can correspond up to 25% of the total thickness (height) of the foam. - a foam elasticity Et in the lower foam layer that is at least 3 times greater, preferably 3 to 10 times greater than in the upper foam layer and where foam elasticity corresponds to formula [2]: With ωη = the natural frequency, m = a fixed mass, h = the thickness of the foam sample A = the cross-sectional area of ​​the foam sample According to embodiments, the molded flexible polyurethane comprising the foam of the invention has a polymer elasticity E in the lower foam layer that is at least 2 times greater, preferably from 2 to 8 times greater than in the upper foam layer and where the polymer elasticity corresponds to formula [1]: Ev=Ei [1] With Pf - The relative density (R) defined as 7? = — Pp - The polymeric density of polyurethane (pp) = 1200 kg / m3, - The density of polyurethane foam (pf) being measured according to ISO 845 According to embodiments, the molded flexible polyurethane comprising the foam of the invention has a foam density p< in the lower foam layer that is 10% to 40% greater than in the upper foam layer. According to one embodiment, the molded flexible polyurethane comprising the foam of the invention has a polymer elasticity Epen in the lower foam layer that is at least 2 times greater, preferably 2 to 8 times greater than in the upper foam layer and wherein the polymer elasticity corresponds to formula [1] and a hardness gradient has a foam density pf in the lower foam layer that is 10% to 40% greater than in the upper foam layer. According to one embodiment, the molded flexible polyurethane comprising the foam of the invention has a polymer elasticity Epen the top layer that is less than the polymer elasticity Epen the core (middle section) of the foam. According to embodiments, the molded flexible polyurethane comprising the foam of the invention has a foam density pi in the lower half of the foam that is greater than the foam density pi in the upper half of the foam. According to embodiments, the molded flexible polyurethane comprising the foam of the invention has a polymer elasticity Epen in the top layer that is less than the polymer elasticity Epen in the core (middle section) of the foam and a foam density pt in the lower half of the foam that is greater than the foam density pt in the upper half of the foam. Surprisingly, it has been found that a combination of a well-defined reactive foam formulation and well-defined process conditions will result in the flexible foam according to the invention, which has a hardness gradient, the hardness gradient being the result of the foam elasticity gradient arising from the polymer elasticity gradient and / or the density gradient. The processing conditions according to the invention used to manufacture the flexible foam having the hardness gradient according to the invention in a mold comprise the use of a well-defined temperature difference (ΔT) between the reactive foam formulation used (Tchemicals) and the temperature of the mold (Tmold). To achieve the well-defined temperature difference ΔT, either the mold is heated or the reactive foam formulation is cooled, or alternatively, both the mold and the reactive foam formulation are heated and cooled. According to certain embodiments, the temperature difference ΔT between the initial reactive foam formulation used (Chemical Products) and the mold temperature (Mold) is at least 25-30°C, more preferably at least 30-50°C, and more preferably at least 35-55°C. Compared to prior art processing conditions, the invention addresses a much larger temperature difference ΔT between the initial reactive foam formulation used (Chemical Products) and the mold temperature (Mold). Prior art processing typically employs a temperature difference ΔT between the initial reactive foam formulation used (Chemical Products) and the mold temperature (Mold) of 10-20°C, preferably using a ΔT of approximately 15°C. According to the modalities, the minimum temperature of the initial reactive foam formulation used (Tchemicals) is 10-15sC, preferably Tchemicals is close to room temperature. According to the specifications, the mold temperature (Tmoid) is at least 50°C and less than 100°C, preferably the Tmoid is in the range of 55°C to 70°C, more preferably in the range of 60°C to 70°C. Most preferably the Tmoid is approximately 65°C. According to the modalities, the predefined temperature of the mold (Tmoide) is achieved by heating at least the lower part of the mold, preferably the entire mold is heated. The reactive foam formulation according to the invention used to manufacture the flexible foam having a hardness gradient according to the invention comprises the use of a well-defined foam formulation comprising at least one reactive isocyanate composition (a) and one polyisocyanate composition (b). The polyisocyanate composition (b) according to the invention comprises diphenylmethane diisocyanate (MDI) and homologues thereof having an isocyanate functionality of 3 or more (polymeric MDI), wherein the amount of 2,4'-methylene diphenyl diisocyanate (2,4 MDI) is in the range of 0-2% by weight, preferably in the range of 0-10% by weight calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition, the remaining polyisocyanate compounds being polymeric MDI and 4,4'-methylene diphenyl diisocyanate (4,4 MDI). An example of a commercially available polyisocyanate composition is Huntsman's Suprasec® 4801. According to one embodiment, the polyisocyanate composition (b) of the invention is pre-reacted (i.e., prepolymerized) with a polyol, wherein the amount of this polyol reacted in the polyisocyanate composition (b) is in the range of 0–40% by weight, preferably in the range of 0–30% by weight, and more preferably in the range of 0–20% by weight, calculated on the total weight of the polyisocyanate composition (b). The isocyanate-reactive composition (a) can be used as the polyol to form the prepolymerized polyisocyanate composition (b). Alternatively, the polyols used to form the prepolymerized polyisocyanate composition (b) are similar to and / or a selection of the polyols used in the isocyanate-reactive composition (a). According to embodiments, the polyisocyanate composition (b) is a composition comprising diphenylmethane diisocyanate (MDI) and prepolymers having free isocyanate groups consisting of MDI, preferably the polyisocyanate composition (b) is a composition comprising at least 40%, preferably at least 50% by weight of 4,4'-diphenylmethane diisocyanate (4,4-MDI). Preferably, the polyisocyanate composition (b) has an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to embodiments, the polyisocyanate composition (b) is a prepolymer composition wherein the polyisocyanate composition (b) of the invention is pre-reacted (i.e., pre-polymerized) with a polyol. The prepolymer composition preferably comprises 0-12 wt%, preferably 0-10 wt% of 2,4'-methylene diphenyl diisocyanate (2,4 MDI) and comprising at least 40 wt%, preferably at least 50 wt% of 4,4'-diphenylmethane diisocyanate (4,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition and an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to embodiments, the polyisocyanate composition (b) is a prepolymer composition wherein the polyisocyanate composition (b) of the invention is pre-reacted (i.e., prepolymerized) with a polyol. The prepolymer composition preferably comprises 0-12 wt%, preferably 0-10 wt%, of methylene diphenyl 2,4'-diisocyanate (2,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition and a value of NCO in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to embodiments, the polyisocyanate composition (b) is a composition comprising 0-12% by weight, preferably 0-10% by weight of methylene diphenyl 2,4'-diisocyanate (2,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition and an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to the modalities, the polyisocyanate composition (b) has an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%. According to the modalities, the polyisocyanate composition (b) has an NCO value in the range of 20 to 25.5%, preferably in the range of 23 to 25.5%. According to embodiments, the reactive isocyanate composition (a) according to the invention comprises: a) a polyether polyol (a1) having an oxypropylene (PO) content of 51-100% by weight, an oxyethylene (EO) content of 0-49% by weight, preferably an oxyethylene (EO) content of at most 20% by weight calculated on the total weight of the polyol (a1), an average nominal hydroxyl functionality of 2-4 and an average molecular weight of 2000-7000, and b) 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 the polyol (a2) in the isocyanate reactive composition (a) is in the range of 0 to 20% by weight, preferably in the range of 0 to 10% by weight, more preferably in the range of 0 to 5% by weight calculated on the total weight of the isocyanate reactive composition (a), and c) optionally a filled polyether polyol (a3), also called a polymeric polyol, wherein the weight ratio of the polyol (a3) ​​in the isocyanate reactive composition (a) is in the range of 0 to 30% by weight, preferably in the range of 0 to 20% by weight calculated on the total weight of the isocyanate reactive composition (a). According to embodiments, the reactive isocyanate composition (a) according to the invention comprises a proportion of primary hydroxyl groups of more than 70%. The polyether polyols (a1), (a2), and (a3) ​​used in the manufacture of the flexible foam according to the present invention are obtained by the polymerization of propylene oxide and optionally ethylene oxide in the presence, where necessary, of polyfunctional initiators. The polyether polyols are also referred to as polyoxyethylene-polyoxypropylene polyols. Suitable initiator compounds used to prepare the polyols contain a plurality of active hydrogen atoms and include water, butanediol, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, ethanolamine, diethanolamine, triethanolamine, cyclohexane-dimethanol, glycerol, trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol, and sorbitol. Mixtures of initiators and / or cyclic oxides may be used.Polyoxyethylene polyoxypropylene polyols are obtained by the simultaneous or sequential addition of ethylene and propylene oxides to initiators as fully described in the prior art. The most preferred examples to polyether polyols (a1) are polyoxypropylene polyols and polyoxyethylene polyoxypropylene polyols, which have an average nominal hydroxyl functionality of 2–4, an average molecular weight of 2000–7000, and an oxyethylene content of at most 20% by weight, calculated on the weight of the polyol. Commercially available examples include Daltocel® F428 and Daltocel® F435 from Huntsman, Alcupol® F4811 from Repsol, Voranol® CP3322, NC 700, and HL 400 from Dow, Caradol® SC 48-08 from Shell, and Arcol® 1374 from Bayer. The most preferred examples of commercially available polyether polyols (a2) are Huntsman's Daltocel® F442, Daltocel® F444, and Daltocel® F555. According to embodiments, the reactive foam formulation according to the invention further comprises additives such as blowing agents, catalysts, chain extenders and other additives such as flame retardants, fillers, surfactants, ... Preferably the additives are added to the isocyanate reactive composition (a) before combining the isocyanate reactive composition (a) with the polyisocyanate composition (b). According to certain embodiments, the reactive foam formulation according to the invention comprises blowing agents, the blowing agent preferably comprising water. In preferred embodiments, the blowing agent is selected from water in an amount of 0.5 to 10% by weight, preferably in an amount of 1 to 5% by weight calculated on the total weight of all ingredients present in the reactive isocyanate composition (a) used to form the reactive foam formulation according to the invention. According to various embodiments, the reactive foam formulation according to the invention comprises a chain extender. Preferred chain extenders are reactive isocyanate chain extenders having 2-8 reactive hydrogen atoms and a molecular weight up to 999. Examples include butanediol, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, ethanolamine, diethanolamine, triethanolamine, cyclohexane dimethanol, glycerol, trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol, sorbitol, and polyoxyethylene diols having an average molecular weight between 200 and 600, and mixtures of these compounds. According to certain embodiments, the reactive foam formulation according to the invention comprises a surfactant, preferably the surfactant being used in an amount of 0.1-5 and preferably 0.2-2% by weight calculated on the total weight of all ingredients present in the isocyanate reactive composition (a) used to form the reactive foam formulation according to the invention. The surfactant is preferably a polysiloxane polymer and more particularly a polyoxyalkylene polysiloxane polymer, preferably having a molecular weight of 5000-60000. Preferred examples of commercially available surfactants are Tegostab® B8734LF and Tegostab® B8738 and Tegostab® B8745 from Evonik. According to certain embodiments, the reactive foam formulation according to the invention comprises catalysts. In preferred embodiments, the catalysts are selected from well-known catalysts used in the field of polyurethane foams. Examples of well-known catalysts include amine-based catalysts and tin catalysts. Generally, catalysts used to manufacture flexible polyurethane foams are roughly classified into gelling catalysts that accelerate the resinification of the polyurethane and blowing catalysts that accelerate the foaming of the polyisocyanate component. A preferred gelling catalyst is a tertiary amine catalyst that particularly accelerates the reaction between a polyisocyanate and a polyol, and is not particularly limited to such catalysts. Examples of such catalysts include triethylenediamine, 1,8-diazabicyl[5.4.0]undecene-7, imidazoles as reactive 1-methyl amine catalysts, which can be selected from known prior art tertiary amine catalysts capable of promoting the reaction between a polyisocyanate and a polyol, thereby forming a urethane linkage, meaning the catalyst can be chemically incorporated into the polyurethane matrix. Preferably, the tertiary amine catalysts have at least one reactive isocyanate hydrogen atom and preferably one or more primary and / or secondary amine groups and / or one or more hydroxyl groups. Examples of suitable reactive tertiary amine catalysts are the following catalysts: - Ν,Ν-3-dimethylaminopropylamine (Jeffcat® DMAPA from Huntsman), - Ν,Ν-dimethylethanolamine (Jeffcat® DMEA from Huntsman), - Ν,Ν-dimethylaminoethoxyethanol (Jeffcat® ZR70 from Huntsman), - Ν,Ν,Ν'-trimethyl-N'-hydroxyethyl-bisaminoethyl ether (Jeffcat® ZF10 from Huntsman), - N,N-bis-(3-dimeth¡lam¡nopropyl)-N-isopropanolamine (Jeffcat® ZR50 by Huntsman), - N-(3-dimeth¡lam¡nopropyl)-N,N-diisopropanolam¡na (Jeffcat® DPA by Huntsman), - N,N,N'-tr¡meth¡l-N'-(hydrox¡et¡l)et¡lend¡am¡na (Jeffcat® Z110 by Huntsman), - tetramethyliminobispropylamine (Jeffcat® Z130 by Huntsman), - N-[2-[2-(dimethylamino)ethoxy]ethyl]-N-methyl-1,3-propanodiamine (Dabco® NE 300 from Evonik), - 2-(dimethylamino-)ethane-1-ol (Jeffcat® TD 20 by Huntsman), - tetramethyliminobispropylamine (Jeffcat® Z 130 by Huntsman), - 2-(2-(2-dimethylam¡noethoxy¡-)ethyl methyl am¡no-)ethanol (Dabco® NE 1061 from Evonik), - bis(d¡met¡lam¡nomethyl-)phenol (Dabco® TMR 30 de Evonik). Furthermore, a foaming process is described for manufacturing the flexible foam having a hardness gradient according to the invention, the process comprising reacting an isocyanate index in the range 70-130, preferably in the range 75-110, more preferably in the range 75-100, the reactive foam formulation according to the invention in a mold using the processing conditions according to the invention in the mold. According to embodiments of the invention, the process for manufacturing the flexible foam having a hardness gradient according to the invention comprises at least the following steps: i. premixing the reactive isocyanate composition (a) according to the invention with chain extenders, catalysts, blowing agents and other additives, and then ii. mixing the polyisocyanate composition (b) according to the invention with the premixed reactive isocyanate composition (a) obtained in step i) to an isocyanate index in the range 70-130, preferably in the range 75-110, more preferably in the range 75-100 obtained to obtain a reactive foam formulation, and then iii. melting the reactive foam formulation obtained in step ii) in a mold using the processing conditions according to the invention to obtain the flexible foam having a hardness gradient, and then iv. demolding the obtained flexible foam having a hardness gradient. According to embodiments of the invention, the process for manufacturing the flexible foam having a hardness gradient according to the invention comprises at least the following steps: i. mixing a polyisocyanate composition (b) with the premixed reactive isocyanate composition (a) obtained in step i. to an isocyanate index in the range 70-130, preferably in the range 75-110, more preferably in the range 75-100 obtained to obtain the reactive foam formulation, and then iii. melting the reactive foam formulation obtained in step iii. into a mold to obtain flexible foam having a hardness gradient, and then iii. demolding the obtained flexible foam having a hardness gradient characterized in - the isocyanate reactive composition (a) comprises at least 50% by weight, preferably at least 70% by weight calculated on the total weight of the isocyanate reactive composition (a), 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, - the polyisocyanate composition (b) comprises 0-12% by weight, preferably 0-10% by weight of methylene diphenyl 2,4'-diisocyanate (2,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition, the remaining polyisocyanate compounds being polymeric MDI and variants thereof, and - step iii. is carried out so that there is a temperature difference (ΔT) of at least 25-30°C between the temperature of the reactive foam formulation (Tchemicals) and the temperature of the mold (Tmold). According to the modality, the reactive composition of isocyanate (a) is first premixed with chain extenders, catalysts, blowing agents and other additives. According to the modalities, step i¡¡. is carried out so that there is a temperature difference (ΔT) between the initial reactive foam formulation (Tchemicals) and the mold temperature (Tmold) of at least 25-30°C, more preferably at least 30-50°C, the most preferred temperature difference ΔT is at least in the range of 35-55°C. According to embodiments, the isocyanate reactive composition (b) in step i. further comprises 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 the polyol (a2) and the amount of polyether polyol (a2) in the isocyanate reactive composition (a) is in the range of 0 to 20% by weight, preferably in the range of 0 to 10% by weight, more preferably in the range of 0 to 5% by weight calculated on the total weight of the isocyanate reactive composition (a). According to embodiments, the isocyanate reactive composition (b) in step i. further comprises a filled polyether polyol (a3), also called a polymeric polyol, wherein the weight ratio of the polyol (a3) ​​in the isocyanate reactive composition (a) is in the range of 0 to 30% by weight, preferably in the range of 0 to 20% by weight calculated on the total weight of the isocyanate reactive composition (a). According to embodiments of the invention, the mixing step of a polyisocyanate composition (b) with the premixed reactive isocyanate composition (a) is carried out using a 2-component high-pressure mixing system or a 2-component dynamic mixing system. According to embodiments of the invention, step ii. (casting the reactive foam formulation obtained in step ii.) into a mold is carried out in such a way that the degree of overpacking of the mold is kept low, preferably the overpacking is such that the calculated overpacking ratio of the molded density to the free-rising density is in the range 1-1.5. This means that the molding process (foam formation) takes place until the mold is full. According to embodiments of the invention, step ii. (casting the reactive foam formulation obtained in step ii.) is carried out such that the reactive foam formulation is inserted into the mold at an angle of approximately 30 degrees (measured from the bottom plate of the mold) so that the foam flows down the slope. This means that the surface of the mold is horizontal and the inlet of the reactive foam formulation is at a 30-degree incline, thus promoting the vertical rise of the foam. Furthermore, the invention describes the use of flexible molded foam according to the invention in automotive seats, mattresses, furniture, automotive under-carpets, and dashboard insulation. BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates the configuration of the tool used to measure the natural frequency ωη of a foam sample and the Figure 1b illustrates that the natural frequency ωη of the system is the frequency at which the transmissibility, the ratio of the accelerations measured by the sensors, is at its maximum. Figure 2a illustrates the processing conditions used to prepare Comparative Example 1 not in accordance with the invention, where the reactive foam formulation has a temperature of 30°C and the mold has a temperature of 45°C, so that the temperature difference (ΔT) between the reactive foam formulation (Tchemicals) and the temperature of the mold (Tmold) is approximately 15°C. Figure 2b illustrates the processing conditions according to the invention used to manufacture a flexible foam having a hardness gradient (corresponding to Example 1) where the reactive foam formulation has a temperature of 20°C and the mold has a temperature of 65°C so that the temperature difference (ΔT) between the reactive foam formulation (Tchemicals) and the temperature of the mold (Tmold) is approximately 45°C. Figure 3a illustrates the foam elasticity Et obtained in the different layers in a foam according to the invention (example 1) and a foam not according to the invention (Comparative Example 1). Figure 3b illustrates the polymer elasticity Ep obtained in the different layers in a foam according to the invention (example 1) and a foam not according to the invention (Comparative Example 1). Figure 4a illustrates the foam elasticity Et obtained in the different layers of a foam MA / a / ZUZI / UIOUÜJ • DELA, cross-linker • Tegostab® B8734LF, surfactant • Tegostab® B8745, surfactant • Tegostab® KE 810L, surfactant • Tegostab® B8738, surfactant • Water. Examples 1 and 2 according to the invention and Comparative Examples 1 and 2 Examples 1 and 2 of flexible polyurethane foam according to the invention, having a hardness gradient, were prepared by mixing a polyisocyanate composition (b) and a reactive isocyanate composition (a) to form a reactive foam formulation according to the invention. This reactive foam formulation, having a temperature approximately at room temperature, was filled into a mold where the temperature difference (ΔT) between the temperature of the reactive foam fermentation (Tchemicals) and the temperature of the mold (Tmold) is approximately 45°C. When Example 1 was repeated using the same reactive foam formulation as in Example 1 but with a temperature difference (ΔT) between the temperature of the reactive foam formulation (Tchemicals) and the temperature of the mold (Tmold) that is not in accordance with the invention, the resulting foam (Comparative Example 1) did not have a hardness gradient as defined in the invention. The reactive foam formulation in Comparative Example 1 has a temperature of 30°C and the mold was at a temperature of 45°C, so the temperature difference (ΔT) between the temperature of the reactive foam formulation (Tchemicals) and the temperature of the mold (Tmold) is approximately 15°C, which is insufficient to achieve the flexible foam with a hardness gradient according to the invention. Comparative Example 2 comprises an excessive amount of 2,4'-MDI, and its polyisocyanate composition used to prepare the reactive foam formulation did not result in a foam with a stiffness gradient according to the invention. In Comparative Example 2, the polyisocyanate composition comprises 15 wt% of 2,4'-MDI, calculated on the total weight of all isocyanate compounds in the polyisocyanate composition. The composition of the reactive foam formulations used to prepare Example 1, Example 2, Comparative Example 1, and Comparative Example 2 is illustrated in Table 1. Table 1 IVIA / a / ZUZ l / UI DUOJ Chemical Products Example 1 % by weight Example 2 % by weight Comparative Example 1 % by weight Comparative Example 2 % by weight Polyisocyanate Composition ib} Suprasec® 2525 100 100 Suprasec® 4801 100 Suprasec® 7007 100 Isocyanate Reactive Composition (ia) Daltocel® F428 81.6 95.5 81.6 95.5 Daltocel® F526 3.1 3.1 Alcupol® P-2321 10 10 Jeffcat® DPA 0.62 0.62 Jeffcat® DMEA 0.2 0.2 Jeffcat® LED204 36% 0.15 0.15 Jeffcat® ZF-10 0.1 0.1 DELA 0.26 0.26 Tegostab® B8738 0.2 0.2 Tegostab® B8745 0.4 0.4 Water 3.1 3.1 3.1 3.1 Isocyanate Index 80 90 80 90 MA / a / ZUZI / UIOUÜJ The properties obtained by Examples 1 and 2 according to the invention and by Comparative Examples 1 and 2 are indicated in Tables 2, 3 and 4. Table 2 compares the foam elasticity ratio (foam elasticity / average foam elasticity) of different foam layers. In Examples 1 and 2, there is a clear increase in foam elasticity from the top layer to the bottom layer. In Comparative Examples 1 and 2, the top and bottom layers have higher foam elasticity than the middle layers. Table 3 compares the foam density ratio (foam density / average foam density) of different foam layers. In Examples 1 and 2, the bottom layer has a higher foam density than the other layers. In Comparative Examples 1 and 2, the top and bottom foam layers have a higher foam density than the foam core. Table 4 compares the polymer elasticity ratio (polymer elasticity / average polymer elasticity) of different foam layers. The polymer elasticity of each layer is derived from the foam elasticity according to the formula in the definition of polymer elasticity. In Example 1, the top two layers have a much lower polymer elasticity than the other two layers. In Example 2, the polymer elasticity of the top layer is much lower than that of the other layers. In Comparative Examples 1 and 2, there is limited variation in the polymer elasticity of each layer.From those results in Example 1, the variation in the elasticity of the foam comes mainly from a variation in the polymeric elasticity in the foam with a second effect being the variation in density, whereas in Comparative Examples 1 and 2 the variation in the elasticity of the foam in the upper and lower layer comes from the variation in density. Figures 3a and 3b illustrate the foam elasticity and polymer elasticity in the different layers in the foams obtained from Example 1 and the Comparative Example. Figure 4a illustrates the foam elasticity Et obtained in the different layers in a foam according to the invention (Example 2) and a foam not in accordance with the invention (Comparative Example 2). Figure 4b illustrates the polymer elasticity Ep obtained in the different layers in a foam according to the invention (Example 2) and a foam not according to the invention (Comparative Example 2). Figure 5a illustrates the foam density (pt) obtained in the different layers in foams according to the invention by Example 1 and Comparative Example 1. Figure 5b illustrates the foam density (pt) obtained in the different layers in foams according to the invention for Example 2 and Comparative Example 2. To analyze the properties of the foam, the foam sample was divided into different layers as illustrated in Figure 6. Table 2 ινΐΛ / a / zuz ι / υ i ouoj Layer Foam elasticity ratio of Example 1 Foam elasticity ratio of Comparative Example 1 Foam elasticity ratio of Example 2 Foam elasticity ratio of Comparative Example 2 1 (top layer) 0.2 1.3 0.4 1.2 2 0.6 0.7 0.9 0.6 3 1.4 0.7 1.2 0.7 4 (bottom layer) 1.7 1.3 1.4 1.3 Table 3 Layer Density Ratio of Example 1 Density Ratio of Comparative Example 1 Density Ratio of Example 2 Density Ratio of Comparative Example 2 1 (top layer) 1.0 1.1 0.9 1.1 2 1.0 0.8 0.9 0.8 3 1.0 0.8 1.0 0.8 4 (bottom layer) 1.1 1.0 1.2 1.0 Table 4 Layer Polymer Elasticity Ratio of Example 1 Polymer Elasticity Ratio of Comparative Example 1 Polymer Elasticity Ratio of Example 2 Polymer Elasticity Ratio of Comparative Example 2 1 (top layer) 0.3 1.0 0.5 1.0 2 0.7 0.9 1.2 0.9 3 1.6 0.9 1.3 0.9 4 (bottom layer) 1.4 1.1 1.0 1.1 ινΐΛ / a / zuz ι / ui ouoj

Claims

1. A foam comprising molded flexible polyurethane with a hardness gradient, wherein the foam is characterized in that it comprises at least: - an upper layer having a thickness (height) corresponding approximately 25% of the total thickness (height) of the foam, - a lower layer having a thickness (height) corresponding approximately 25% of the total thickness (height) of the foam, - a foam elasticity Et in the lower foam layer that is at least 3 times greater, preferably from 3 to 10 times greater, than in the upper foam layer, and wherein the foam elasticity corresponds to the formula [2]: ML / a / ZUZ 1 / U1 OUOJ with ωη = the natural frequency, m = a fixed mass, h = the thickness of the foam sample, A = the cross-sectional area of ​​the foam sample, and wherein the foam is manufactured using a reactive foam formulation, the reactive foam formulation being blended to form an isocyanate index in the range 70-130,preferably in the range of 75-110, more preferably in the range of at least 75-100: - A reactive isocyanate 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, preferably at most 20% by weight calculated on the total weight of the polyol (a1), 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 the polyol (a2) in the reactive isocyanate composition (a) is in the range of 0 to 20% by weight, preferably in the range of 0 to 10% by weight, more preferably in the range of 0 to 5% by weight calculated on the total weight of the reactive isocyanate composition (a),and - a polyisocyanate composition (b) having an NCO value in the range of 21 to 27%, preferably in the range of 23 to 25.5%., 2. The molded flexible foam according to claim 1, further characterized in that it has a polymer elasticity Ep in the lower foam layer that is at least 2 times greater, preferably from 2 to 8 times greater, in the upper foam layer, and wherein the polymer elasticity corresponds to formula [1]: [1] With pf R = — - The relative density (R) defined as Pp - The polymer density of the polyurethane (pP) = 1200 kg / m3, - The density of polyurethane foam (pt) being measured according to ISO 845 3. The molded flexible foam according to any of claims 1 or 2, further characterized in that it has a foam density pf in the lower foam layer which is 10% to 40% greater than in the upper foam layer.

4. The molded flexible foam according to any of the preceding claims, further characterized in that it has a polymer elasticity Ep in the lower foam layer which is at least 2 times greater, preferably 2 to 8 times greater than in the upper foam layer and wherein the polymer elasticity corresponds to formula [1] and a hardness gradient has a foam density pf in the lower foam layer which is 10% to 40% greater than in the upper foam layer.

5. The molded flexible foam according to any of the preceding claims, further characterized in that it has a polymeric elasticity Ep in the top layer which is less than the polymeric elasticity EP in the core (middle section) of the foam.

6. The molded flexible foam according to any of the preceding claims, further characterized in that it has a foam density p< in the lower half of the foam which is greater than the foam density pt in the upper half of the foam.

7. The flexible molded foam according to any of the preceding claims, further characterized in that it has a polymer elasticity Ep in the top layer that is less than the polymer elasticity Ep in the middle section of the foam and a foam density pt in the lower half of the foam that is greater than the foam density pf in the upper half of the foam.

8. A reactive foam formulation for manufacturing molded flexible foam according to any of the preceding claims, characterized in that the polyisocyanate composition (b) comprises 0-12% by weight, preferably 0-10% by weight of 2,4' methylene diphenyl diisocyanate (2,4 MDI) calculated on the total weight of all polyisocyanate compounds in the polyisocyanate composition.

9. The reactive foam formulation for manufacturing the molded flexible foam according to any of the preceding claims, further characterized in that the formulation further comprises a filled polyether polyol (a3) ​​wherein the weight ratio of the polyol (a3) ​​in the reactive isocyanate composition (a) is in the range of 0 to 30% by weight, preferably in the range of 0 to 20% by weight calculated on the total weight of the reactive isocyanate composition (a).

10. The reactive foam formulation for manufacturing the molded flexible foam according to any of the preceding claims, further characterized in that the polyisocyanate composition (b) is pre-reacted (i.e., pre-polymerized) with a polyol and the amount of polyol that reacted in the polyisocyanate composition (b) is in the range 0-40% by weight, preferably in the range 0-30% by weight, more preferably in the range 0-20% by weight, calculated on the total weight of the polyisocyanate composition (b).

11. The reactive foam formulation for manufacturing molded flexible foam according to any of the preceding claims, further characterized in that it also comprises additives such as blowing agents, catalysts, chain extenders and other additives such as flame retardants, fillers, surfactants,...

12. The reactive foam formulation for manufacturing the molded flexible foam according to any of the preceding claims, further characterized in that it comprises blowing agents, the blowing agent comprising at least water, and the amount of water used is from 0.5 to 10% by weight, preferably 1 to 5% by weight calculated on the total weight of all ingredients present in the reactive isocyanate composition (a) used to form the reactive foam formulation according to the invention.

13. A process for manufacturing molded flexible foam according to any of the preceding claims, characterized in that the process comprises at least the steps of: i. mixing the polyisocyanate composition (b) with the reactive isocyanate composition (a) at an isocyanate index in the range of 70-130, preferably in the range of 75-110, more preferably in the range of 75-100 to obtain the reactive foam formulation according to any of claims 9 to 12, and then ii. melting the reactive foam formulation obtained in step i. into a mold to obtain flexible foam having a hardness gradient, and then iii. demolding the obtained flexible foam having a hardness gradient, characterized in that in step iii.It is carried out so that there is a temperature difference (ΔT) of at least 25-30°C between the temperature of the reactive foam formulation (Tchemical products) and the temperature of the mold (Tmold).

14. The process according to claim 13, further characterized in that the temperature difference ΔT between the initial reactive foam formulation used (Tchemicals) and the temperature of the mold (Tmold) is at least 25-30°C, more preferably at least 30-50°C, more preferably the temperature difference ΔT is at least in the range of 35-55°C.

15. The process according to any of claims 13 or 14, further characterized in that the minimum temperature of the reactive foam formulation used (Tchemicals) is 10-15°C, preferably Tchemicals is around ambient temperature and the temperature of the mold (Tmold) is at least 50°C and less than 100°C, preferably Tmold is in the range of 55°C to 70°C, more preferably in the range of 60°C to 70°C.

16. Use of the molded flexible foam according to any of claims 1 to 7, such as automotive seats, mattresses, furniture, automotive under-carpets and dashboard insulation.