Patent Publication Number: US-6043292-A

Title: Process for making flexible polyurethane foams

Description:
FIELD OF THE INVENTION 
     The present invention is concerned with the preparation of rigid and flexible polyurethane foams using an inert gas as blowing agent. 
     BACKGROUND OF THE INVENTION 
     Conventional flexible polyurethane foams are widely known. Such foams show a relatively high resilience (ball rebound), a relatively low modulus, a relatively high sag factor and a relatively low hysteresis loss. Such foams further show a major glass-rubber transition below ambient temperature, generally in the temperature range of -100° C. to -10° C. The commonly applied relatively high molecular weight polyether and polyester polyols in such foams are responsible for the sub-ambient glass transition temperature (Tg s ). These polyether and polyester polyols are often referred to as soft segments. Above Tg s  the foam displays its typical flexible properties until softening and/or melting of the isocyanate-derived urethane/urea clusters (&#34;hard domains&#34;) takes place. This softening and/or melting temperature (Tg h  and/or Tm h ) often coincides with the onset of thermal degradation of polymer segments. The Tg h  and/or Tm h  for flexible polyurethane foams is generally higher than 100° C., often even exceeding 200° C. At the Tg s  a sharp decrease of the modulus of the flexible foam is observed. Between Tg s  and Tg h  /Tm h  the modulus remains fairly constant with increasing temperature and at Tg h  /Tm h  again a substantial decrease of the modulus may take place. A way of expressing the presence of Tg s  is to determine the ratio of the Young&#39;s storage modulus E&#39; at -100° C. and +25° C. as per Dynamic Mechanical Thermal Analysis (DMTA measured according to ISO/DIS 6721-5). For conventional flexible polyurethane foams the ##EQU1## 
     Another feature of Tg s  by DMTA (ISO/DIS 6721-5) is that for conventional flexible polyurethane foams the maximum value of the ratio of ##EQU2## over the -100° C./+25° C. temperature range varies from 0.20-0.80 in general. The Young&#39;s loss modulus E&#34; is measured by DMTA (ISO/DIS 6721-5) as well. 
     Conventional flexible foams are made by reacting a polyisocyanate and a relatively high molecular weight isocyanate reactive polymer, often a polyester or polyether polyol, in the presence of a blowing agent and optionally further using limited amounts of relatively low molecular weight chain extenders and cross-linkers and optionally using additives like catalysts, surfactants, fire retardants, stabilisers and antioxidants. The relatively high molecular weight isocyanate reactive polymer in general represents the highest weight fraction of the foam. Such flexible foams may be prepared according to the one-shot, the quasi- or semi-prepolymer or the prepolymer process. Such flexible foams may be moulded foams or slabstock foams and may be used as cushioning material in furniture and automotive seating and in mattresses, as carpet backing, as hydrophilic foam in diapers and as packaging foam. Further they may be used for acoustic applications, e.g. sound insulation. Examples of prior art for these conventional flexible foams are EP-10850, EP-22617, EP-111121, EP-296449, EP-309217, EP-309218, EP-392788 and EP-442631. 
     Conventional rigid foams are made in a similar way with the proviso that often the polyisocyanates have a higher isocyanate functionality, the amount of high molecular weight polyols used is lower and the amount and functionality of the cross-linkers is higher. 
     WO92/12197 discloses an energy-absorbing, open-celled, water-blown, rigid polyurethane foam obtained by reacting a polyurethane foam formulation, comprising water which acts as a blowing agent and a cell-opener, in a mould wherein the cured foam has a moulded density of about 32 to 72 kg/m 3  and a crush strength which remains constant from 10 to 70% deflection at loads of less than 70 psi. The foams have minimal spring back or hysteresis. 
     GB2096616 discloses a directionally flexibilized, rigid, closed-cell plastic foam. The rigid foams are flexibilized in order to use them for e.g. pipe-insulation. Cells should remain closed. 
     U.S. Pat. No. 4,299,883 discloses a sound-absorbent material made by compressing a foam having closed cells to such an extent that the foam recovers to 50-66% of its original thickness. By the compression the cells are ruptured and the foam becomes flexible and resilient; it may replace felt. The disclosure mainly refers to polycarbodiimide foams. 
     EP561216 discloses the preparation of foam boards having improved heat insulation properties, wherein the foam has anisotropic cells having a length ratio of the long and the small axis of 1.2-1.6 and a density of 15-45 kg/m 3  and wherein the cells have been crushed in the direction of the plate thickness. The disclosure actually refers to polystyrene boards. 
     EP641635 discloses a process for preparing foam boards, having a dynamic stiffness of at most 10 MN/m 3 , by crushing a board of 17-30 kg/m 3  density at least twice to 60-90% of its original thickness. Preferably closed-celled polystyrene is used. In the examples it is shown that a polystyrene foam which has been crushed showed a better heat insulation than an uncrushed one. 
     U.S. Pat. No. 4,454,248 discloses a process for preparing a rigid polyurethane foam wherein a partially cured rigid foam is softened, then crushed and re-expanded and fully cured. 
     In copending patent application PCT/EP9601594 a class of flexible polyurethane foams is described such foams having no major glass-rubber transition between -100° C. and +25° C. In more quantitative terms these foams show a ratio E&#39; -100 ° C. /E&#39; +25 ° C. of 1.3 to 15.0, preferably of 1.5 to 10 and most preferably of 1.5 to 7.5. The tan dmax  over the -100° C. to +25° C. temperature range is below 0.2. The apparent core density of such foams may range from 4-30 kg/m 3  and preferably ranges from 4-20 kg/m 3  (measured according to ISO 845). Such foams are made by crushing a rigid foam. 
     In this co-pending application it has further been disclosed to prepare rigid foams and flexible foams using water as blowing agent if desired together with a gas like CO 2 . 
     SUMMARY OF THE INVENTION 
     It has now been found that foams may be made having improved properties by bringing CO 2  into contact with the polyisocyanate before the polyisocyanate is brought into contact with the isocyanate-reactive compounds and the water. The CO 2  improves the air flow through the foam; further a density reduction may be obtained. 
     Consequently the present invention is concerned with the use of CO 2  as air flow improver in the preparation of water-blown polyurethane foams, in particular rigid and flexible polyurethane foams. Further the present invention is concerned with a proces for preparing a flexible foam by reacting a polyisocyanate (1), and isocyanate-reactive compound (2), said compound (2) having an average equivalent weight of at most 374 and an average number of isocyanate-reactive hydrogen atoms of from 2 to 8, an isocyanate-reactive compound (3), said compound (3) having an average equivalent weight of more than 374 and an average number of isocyanate-reactive hydrogen atoms of from 2 to 6 and water to prepare a rigid polyurethane foam and by crushing this rigid polyurethane foam, characterised in that the polyisocyanate is brought into contact with CO 2  before the polyisocyanate is brought into contact with compound 2, compound 3 and water. 
     Further the present invention is concerned with reaction systems comprising the above ingredients. The present invention is also concerned with a process for preparing rigid polyurethane foams using the above ingredients. 
     More in particular the foams according to the present invention are prepared by reacting a polyisocyanate (1), a polyol (2) having a hydroxyl number of at least 150 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 8, a polyol (3) having a hydroxyl number of from 10 to less than 150 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 6 and water to prepare a rigid polyurethane foam and by crushing this rigid polyurethane foam, to prepare a flexible foam, characterised in that the polyisocyanate is brought into contact with CO 2  before the polyisocyanate is brought into contact with compound 2, compound 3 and water. 
     Surprisingly a completely new class of flexible polyurethane foams was found such foams having no major glass-rubber transition between -100° C. and +25° C. and having an air flow resistivity of below 20 and preferably 3-15 and most preferably 3-10 kPa.s/m 2  (ASTM-D3574-86). Further a rigid polyurethane foam was found having an air flow resistivity of 5-40 and preferably 5-30 kPa.s/m 2  (ASTM-D3574-86). In more quantitative terms these foams show a ratio E&#39; -100 ° C. /E&#39; +25 ° C. of 1.3 to 15.0, preferably of 1.5 to 10 and most preferably of 1.5 to 7.5. The core density of such foams may range from 4-30 kg/m 3  and preferably ranges from 4-20 kg/m 3  (measured according to ISO 845). 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the context of the present application a flexible polyurethane foam is a crushed foam having a ball rebound (measured according to ISO 8307) of at least 40%, preferably at least 50% and most preferably 55-85% in at least one of the three dimensional directions and a sag factor (CLD 65/25) of at least 2.0 (measured according to ISO 3386/1). Preferably such flexible foams have a Young&#39;s storage modulus at 25° C. of at most 500 kPa, more preferably at most 350 kPa and most preferably between 10 and 200 kPa (Young&#39;s storage modulus measured by DMTA according to ISO/DIS 6721-5). Further, such flexible foams preferably have a sag factor (CLD 65/25) of at least 3.5 and most preferably 4.5-10 (measured according to ISO 3386/1). Still further such flexible foams preferably have a CLD hysteresis loss (ISO 3386/1) of below 55%, more preferably below 50% and most preferably below 45%. 
     In the context of the present patent application a rigid polyurethane foam is an uncrushed foam having a ball rebound measured in the direction of foam rise of less than 40% (ISO 8307 with the proviso that no preflex conditioning is applied, that only one rebound value per sample is measured and that test pieces are conditioned at 23° C.±2° C. and 50±5% relative humidity) and/or having a CLD 65/25 sag factor measured in the direction of foam rise of less than 2.0 (ISO 3386/1 with the proviso that the sag factor is determined after the first load--unload cycle); these properties both being measured at a core density of the foam of 4-30 kg/m 3  (ISO 845). Preferably the ratio E&#39; -100 ° C. /E&#39; +25 ° C. of such a rigid foam is 1.3-15. If in the present application ISO 8307 and ISO 3386/1 are mentioned in relation to rigid foams they refer to the tests as described above including the provisos. 
     The flexible polyurethane foams according to the present invention are prepared by reacting a polyisocyanate and a polyfunctional isocyanate-reactive polymer under foam forming conditions to prepare a rigid polyurethane foam and by crushing this rigid polyurethane foam. Further the present invention is concerned with the process for preparing such rigid foams and with reaction systems comprising the ingredients for making such foams. 
     In the context of the present invention the following terms have the following meaning: 
     1) isocyanate index or NCO index or index: 
     the ratio of NCO-groups over isocyanate-reactive hydrogen atoms present in a formulation, given as a percentage: ##EQU3## 
     In other words the NCO-index expresses the percentage of isocyanate actually used in a formulation with respect to the amount of isocyanate theoretically required for reacting with the amount of isocyanate-reactive hydrogen used in a formulation. 
     It should be observed that the isocyanate index as used herein is considered from the point of view of the actual foaming process involving the isocyanate ingredient and the isocyanate-reactive ingredients. Any isocyanate groups consumed in a preliminary step to produce modified polyisocyanates (including such isocyanate-derivatives referred to in the art as quasi or semi-prepolymers and prepolymers) or any active hydrogens consumed in a preliminary step (e.g. reacted with isocyanate to produce modified polyols or polyamines) are not taken into account in the calculation of the isocyanate index. Only the free isocyanate groups and the free isocyanate-reactive hydrogens (including those of the water) present at the actual foaming stage are taken into account. 
     2) The expression &#34;isocyanate-reactive hydrogen atoms&#34; as used herein for the purpose of calculating the isocyanate index refers to the total of active hydrogen atoms in hydroxyl and amine groups present in the reactive compositions; this means that for the purpose of calculating the isocyanate index at the actual foaming process one hydroxyl group is considered to comprise one reactive hydrogen, one primary amine group is considered to comprise one reactive hydrogen and one water molecule is considered to comprise two active hydrogens. 
     3) Reaction system: a combination of components wherein the polyisocyanates are kept in one or more containers separate from the isocyanate-reactive components. 
     4) The expression &#34;polyurethane foam&#34; as used herein refers to cellular products as obtained by reacting polyisocyanates with isocyanate-reactive hydrogen containing compounds, using foaming agents, and in particular includes cellular products obtained with water as reactive foaming agent (involving a reaction of water with isocyanate groups yielding urea linkages and carbon dioxide and producing polyurea-urethane foams) and with polyols, aminoalcohols and/or polyamines as isocyanate-reactive compounds. 
     5) The term &#34;average nominal hydroxyl functionality&#34; is used herein to indicate the number average functionality (number of hydroxyl groups per molecule) of the polyol or polyol composition on the assumption that this is the number average functionality (number of active hydrogen atoms per molecule) of the initiator(s) used in their preparation although in practice it will often be somewhat less because of some terminal unsaturation. 
     6) The word &#34;average&#34; refers to number average unless indicated otherwise. 
     The foams according to the present invention are prepared by reacting a polyisocyanate (1), an isocyanate-reactive compound (2), said compound (2) having an average equivalent weight of at most 374 and an average number of isocyanate-reactive hydrogen atoms of from 2 to 8, an isocyanate-reactive compound (3), said compound (3) having an average equivalent weight of more than 374 and an average number of isocyanate-reactive hydrogen atoms of from 2 to 6 and water to prepare a rigid polyurethane foam and by crushing this rigid polyurethane foam. 
     Further the present invention is concerned with reaction systems comprising the above ingredients. The present invention is also concerned with a process for preparing rigid polyurethane foams using the above ingredients. 
     More in particular the foams according to the present invention are prepared by reacting a polyisocyanate (1), a polyol (2) having a hydroxyl number of at least 150 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 8, a polyol (3) having a hydroxyl number of from 10 to less than 150 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 6 and water to prepare a rigid polyurethane foam and by crushing this rigid polyurethane foam. 
     Suitable organic polyisocyanates for use in the process of the present invention include any of those known in the art for the preparation of rigid polyurethane foams, like aliphatic, cycloaliphatic, araliphatic and, preferably, aromatic polyisocyanates, such as toluene diisocyanate in the form of its 2,4 and 2,6-isomers and mixtures thereof and diphenylmethane diisocyanate in the form of its 2,4&#39;-, 2,2&#39;- and 4,4&#39;-isomers and mixtures thereof, the mixtures of diphenylmethane diisocyanates (MDI) and oligomers thereof having an isocyanate functionality greater than 2 known in the art as &#34;crude&#34; or polymeric MDI (polymethylene polyphenylene polyisocyanates), the known variants of MDI comprising urethane, allophanate, urea, biuret, carbodiimide, uretonimine and/or isocyanurate groups. 
     Mixtures of toluene diisocyanate and diphenylmethane diisocyanate and/or polymethylene polyphenylene polyisocyanates may be used. Most preferably polyisocyanates are used which have an average isocyanate functionality of 2.1-3.0 and preferably of 2.2-2.8. 
     Preferably MDI, crude or polymeric MDI and/or liquid variants thereof are used said variants being obtained by introducing uretonimine and/or carbodiimide groups into said polyisocyanates, such a carbodiimide and/or uretonimine modified polyisocyanate having an NCO value of at least 20% by weight, and/or by reacting such a polyisocyanate with one or more polyols having a hydroxyl functionality of 2-6 and a molecular weight of 62-500 so as to obtain a modified polyisocyanate having an NCO value of at least 20% by weight. 
     Isocyanate-reactive compounds (2) include any of those known in the art for that purpose like polyamines, aminoalcohols and polyols. Of particular importance for the preparation of the rigid foams are polyols and polyol mixtures having hydroxyl numbers of at least 150 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 6. Suitable polyols have been fully described in the prior art and include reaction products of alkylene oxides, for example ethylene oxide and/or propylene oxide, with initiators containing from 2 to 8 active hydrogen atoms per molecule. Suitable initiators include: polyols, for example ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butane diol, glycerol, trimethylolpropane, triethanolamine, pentaerythritol, sorbitol and sucrose; polyamines, for example ethylene diamine, tolylene diamine, diaminodiphenylmethane and polymethylene polyphenylene polyamines; and aminoalcohols, for example ethanolamine and diethanolamine; and mixtures of such initiators. Other suitable polyols include polyesters obtained by the condensation of appropriate proportions of glycols and higher functionality polyols with polycarboxylic acids. Still further suitable polyols include hydroxyl terminated polythioethers, polyamides, polyesteramides, polycarbonates, polyacetals, polyolefins and polysiloxanes. Still further suitable isocyanate-reactive compounds include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butane diol, glycerol, trimethylolpropane, ethylene diamine, ethanolamine, diethanolamine, triethanolamine and the other initiators mentioned before. Mixtures of such isocyanate-reactive compounds may be used as well. 
     Isocyanate-reactive compounds (3) include any of those known in the art for that purpose, like polyamines, aminoalcohols and polyols. 
     Of particular importance for the preparation of the rigid foams are polyols and polyol mixtures having a hydroxyl value of 10 to less than 150 and preferably of 15-60 mg KOH/g and an average nominal hydroxyl functionality of from 2 to 6 and preferably of from 2 to 4. These high molecular weight polyols are generally known in the art and include reaction products of alkylene oxides, for example ethylene oxide and/or propylene oxide, with initiators containing from 2 to 6 active hydrogen atoms per molecule. Suitable initiators include: polyols, for example ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butane diol, glycerol, trimethylolpropane, triethanolamine, pentaerythritol and sorbitol; polyamines, for example ethylene diamine, tolylene diamine, diaminodiphenylmethane and polymethylene polyphenylene polyamines; and aminoalcohols, for example ethanolamine and diethanolamine; and mixtures of such initiators. Other suitable polyols include polyesters obtained by the condensation of appropriate proportions of glycols and higher functionality polyols with polycarboxylic acids. Still further suitable polyols include hydroxyl terminated polythioethers, polyamides, polyesteramides, polycarbonates, polyacetals, polyolefins and polysiloxanes. Preferred polyols are the polyether polyols comprising ethylene oxide and/or propylene oxide units and most preferably polyoxyethylene polyoxypropylene polyols having an oxyethylene content of at least 10% and preferably 10-85% by weight. Other polyols which may be used comprise dispersions or solutions of addition or condensation polymers in polyols of the types described above. Such modified polyols, often referred to as &#34;polymer&#34; polyols have been fully described in the prior art and include products obtained by the in situ polymerisation of one or more vinyl monomers, for example styrene and acrylonitrile, in polymeric polyols, for example polyether polyols, or by the in situ reaction between a polyisocyanate and an amino- or hydroxy-functional compound, such as triethanolamine, in a polymeric polyol. 
     The polymer modified polyols which are particularly interesting in accordance with the invention are products obtained by in situ polymerisation of styrene and/or acrylonitrile in poly(oxyethylene/oxypropylene) polyols and products obtained by in situ reaction between a polyisocyanate and an amino or hydroxy-functional compound (such as triethanolamine) in a polyoxyethylene polyoxypropylene polyol. Polyoxyalkylene polyols containing from 5 to 50% of dispersed polymer are particularly useful. Particle sizes of the dispersed polymer of less than 50 microns are preferred. Mixtures of such isocyanate-reactive compounds may be used as well. 
     The relative amount of isocyanate-reactive compound (2) and (3) or polyol (2) and (3) may vary widely and preferably ranges from 0.1:1 to 4:1 (w:w). 
     The relative quantities of the polyisocyanate and the isocyanate-reactive compounds to be reacted may vary within a wide range. In general an isocyanate index will be applied of from 25 to 300, preferably of from 30 to 200 and most preferably of from 40 to 150. 
     In order to prepare a foam water is used as a blowing agent. However if the amount of water is not sufficient to obtain the desired density of the foam any other known way to prepare polyurethane foams may be employed additionally, like the use of reduced or variable pressure, the use of more conventional blowing agents like chlorofluorocarbons, hydrofluorocarbons, hydrocarbons and fluorocarbons, the use of other reactive blowing agents, i.e. agents which react with any of the ingredients in the reacting mixture and due to this reaction liberate a gas which causes the mixture to foam and the use of catalysts which enhance a reaction which leads to gas formation like the use of carbodiimide-formation-enhancing catalysts such as phospholene oxides. Combinations of these ways to make foams may be used as well. The amount of blowing agent may vary widely and primarily depends on the desired density. Water may be used as liquid at below-ambient, ambient or elevated temperature and as steam. 
     As said, the polyisocyanate is brought into contact with CO 2  prior to bringing the polyisocyanate into contact with the isocyanate-reactive compounds and the water. The CO 2  may be brought into contact with the polyisocyanate in the container wherein the polyisocyanate is kept, in the feed-line wherein the polyisocyanateis transported to the mixing device wherein it is mixed with the isocyanate-reactive compounds and the water or in said mixing device prior to mixing the polyisocyanate with the isocyanate-reactive compounds and the water. The CO 2  may be brought into contact with the polyisocyanate by leading it as a gas through the polyisocyanate, by keeping it dissolved and/or as a gas in the polyisocyanate by applying pressure, or by keeping it dissolved and/or as a liquid in the polyisocyanate by applying pressure. 
     The CO 2  may therefore be in contact with the polyisocyanate in the form of a solution, a mixture, an emulsion and/or a dispersion. The amount of CO 2  in the polyisocyanate may vary from 0.1 to 12% by weight and preferably from 0.2 to 6% by weight and most preferably from 0.5 to 3.0% by weight calculated on the weight of the polyisocyanate. 
     The CO 2  may be used together with other inert blowing agents as long as at least 50% and preferably at least 80% and most preferably at least 95% by weight of it is CO 2 . 
     Per 100 parts by weight of polyisocyanate (1), isocyanate-reactive compound (2) and compound (3) or polyol (2) and polyol (3) and water, preferably the amount of compound (2) or polyol (2) ranges from 2-20 parts by weight, the amount of compound (3) or polyol (3) ranges from 5-35 parts by weight and the amount of water ranges from 1 to 17 parts by weight, the remainder being polyisocyanate. This encompasses another aspect of the invention: if a cyclic polyisocyanate and more in particular an aromatic polyisocyanate and most in particular an MDI or polymethylene polyphenylene polyisocyanate is used the content of cyclic and more in particular of aromatic residues in the flexible foam is relatively high as compared to conventional flexible polyurethane foams. The foams according to the invention preferably have a content of benzene rings, derived from aromatic polyisocyanates, which is 30 to 56 and most preferably 35 to 50% by weight based on the weight of the foam. Since polyols, polymer polyols, fire retardants, chain extenders and/or fillers which contain benzene rings may be used, the overall benzene ring content of the flexible foam may be higher and preferably ranges from 30 to 70 and most preferably from 35 to 65% weight as measured by calibrated Fourier Transform Infra Red Analysis. 
     In addition to the polyisocyanate, the isocyanate-reactive compounds and the blowing agent, one or more auxiliaries or additives known per se for the production of polyurethane foams may be used. Such optional auxiliaries or additives include foam-stabilizing agents or surfactants, for example siloxane-oxyalkylene copolymers and polyoxyethylene polyoxypropylene block copolymers, urethane/urea catalysts, for example tin compounds such as stannous octoate or dibutyltin dilaurate and/or tertiary amines such as dimethylcyclohexylamine or triethylene diamine and/or phosphates like NaH 2  PO 4  and Na 2  HPO 4 , and fire retardants, for example halogenated alkyl phosphates such as tris chloropropyl phosphate, melamine and guanidine carbonate, anti-oxidants, anti-static agents, UV stabilisers, anti-microbial and anti-fungal compounds and fillers like latex, TPU, silicates, barium and calcium sulphates, chalk, glass fibers or beads and polyurethane waste material. 
     In operating the process for making rigid foams according to the invention, the known one-shot, prepolymer or semi-prepolymer techniques may be used together with conventional mixing methods and the rigid foam may be produced in the form of slabstock, mouldings including foam in fabric and pour-in-place applications, sprayed foam, frothed foam or laminates with other materials such as hardboard, plasterboard, plastics, paper or metal or with other foam layers. 
     It is convenient in many applications to provide the components for polyurethane production in pre-blended formulations based on each of the primary polyisocyanate and isocyanate-reactive components. In particular, an isocyanate-reactive composition may be used which contains the auxiliaries, additives and the blowing agent in addition to the isocyanate-reactive compounds (2) and (3) in the form of a solution, an emulsion or dispersion. 
     The rigid foam is prepared by allowing the aforementioned ingredients to react and foam until the foam does not rise any more. 
     After rise curing of the foam may be continued as long as desirable. In general a curing period of 1 minute to 24 hours and preferably of 5 minutes to 12 hours will be sufficient. If desired curing may be conducted at elevated temperature. Subsequently the foam may be crushed. It is however preferred to allow the rigid foam obtained to cool down to below 80° C., preferably below 50° C. and most preferably to ambient temperature prior to crushing. The rigid foam (i.e. before crushing) preferably has a core density of 4-30 and most preferably of 4-20 kg/m 3  (ISO 845). 
     The rigid foam (i.e. before crushing) prepared has a substantial amount of open cells. Preferably the cells of the rigid foam are predominantly open. 
     The crushing may be conducted in any known manner and by any known means. The crushing may for instance be conducted by applying mechanical force onto the foam by means of a flat or pre-shaped surface or by applying variations of external pressure. 
     In most cases a mechanical force sufficient to decrease the dimension of the foam in the direction of the crushing by 1-90%, preferably by 50-90% will be appropriate. If desired crushing may be repeated and/or carried out in different directions of the foam. Due to the crushing the ball rebound increases considerably in the direction of the crushing. Due to the crushing the density of the foam may increase. In most cases this increase will not exceed 30% of the density before crushing. 
     Although it is difficult to give more precise directions for the crushing since it will inter alia depend on the density of the foam, the rigidity of the foam, the type of crushing device used, we believe those skilled in the art are sufficiently aware of the phenomenon of crushing of polyurethane foams that they will be able to determine the appropriate crushing manner and means with the above guidance, certainly in the light of the following examples. 
     By crushing the ball rebound is increased at least in the direction of crushing. The increase is at least 10%. 
     After the crushing a novel flexible foam is obtained which has exceptional properties. Despite the fact that the foam is flexible, it does not show a significant change of the Young&#39;s storage modulus E&#39; over a temperature range from -100° C. to +25° C., as described before. The oxygen index of the foam prepared from aromatic polyisocyanates preferably is above 20 (ASTM 2863). Further it shows a Young&#39;s storage modulus at 25° C. of at most 500 kPa, preferably at most 350 kPa, most preferably between 10-200 kPa and a sag factor (CLD 65125, ISO 3386/1) of at least 2.0, preferably at least 3.5 and most preferably of 4.5-10. CLD hysteresis loss values for the foams are below 55% and preferably below 50% (which is calculated by the formula ##EQU4## wherein A and B stand for the area under the stress/strain curve of the loading (A) and unloading (B) as measured according to ISO 3386/1). Still further these foams can be manufactured with a very low or even negative Poisson&#39;s ratio as determined by lateral extension studies under compression of the foams. Finally compression set values of the foams are generally low, preferably below 40% (ISO 1856 Method A, normal procedure). 
     If the Tg h  is not too high the foam might be used in thermoforming processes to prepare shaped articles. Preferably the Tg h  of the foam is between 80 and 180° C., most preferably between 80° C. and 160° C. for such thermoforming applications. Further it was found that foams, which have been made by using a relatively low amount of the polyols having a low molecular weight, show a small or non-visable Tg h  by DMTA (the modulus change at Tg h  is small or the modulus changes gradually until the foam thermally decomposes); such foams however may be used for thermoforming activities as well. 
     Further the foams show good load-bearing properties like compression hardness values without the use of external fillers together with a good resilience, tear strength and durability (fatigue resistance) even at very low densities. In conventional flexible foams often high amounts of filler need to be used to obtain satisfactory load-bearing properties. Such high amounts of fillers hamper the processing due to a viscosity increase. 
     The foams of the present invention may be used as cushioning material in furniture and automotive and aircraft seating and in mattresses, as carpet backing, as hydrophilic foam in diapers, as packaging foam, as foams for sound insulation in automotive applications and for vibration isolation in general. The foam according to the present invention further may be used together with other, conventional flexible foams to form composites, like e.g. in mouldings; such composites may also be made by allowing the ingredients for making the conventional flexible foam to form said foam in a mould in the presence of the foam acccording to the present invention or alternatively by allowing the ingredients for making the rigid foam according to the present invention to form said rigid foam in a mould in the presence of the conventional flexible foam followed by crushing the moulding so obtained. Further the foams according to the present invention may be used as textile cover, as cover for other type of sheets, as carpet underlay or felt-replacement; the so-called flame lamination technique may be applied to adhere the foam to the textile, the carpet or the other sheet. In this respect it is important to note that the foam according to the present invention is suitable to be cut in sheets of limited thickness, e.g. of about 1 cm and less. Still further the foam according to the present invention may be used as insulation material around pipes and containers. 
     The invention is illustrated by the following examples. 
    
    
     EXAMPLE 1 
     Three isocyanate-reactive blends A, B and C were prepared. Blend A was prepared by mixing 20.00 parts by weight (pbw) of an EO/PO polyol having a nominal hydroxyl functionality of 2, diethylene glycol as initiator, an EO content of 20.2% by weight (all tipped except the diethylene glycol initiator) and a hydroxyl value of 30 mg KOH/g and 0.98 pbw of DABCO T9 (catalyst from AIR PRODUCTS, DABCO is a trade mark) and 0.30 pbw of 1-methyl-1-oxo-phospholene (a carbodiimide catalyst from Hoechst). Blend B was prepared by mixing 7.55 pbw of polyethylene glycol having a molecular weight of about 200 and 0.64 pbw of IRGANOX 5057 (an anti-oxidant from Ciba-Geigy, IRGANOX is a trademark). Blend C was prepared by mixing 2.35 pbw of triethylene glycol, 5 pbw of water and 0.11 pbw of NaH 2  PO 4 . 
     A polyisocyanate blend D was made by mixing 60 pbw of a polymeric MDI having an NCO value of 30.7% by weight and an isocyanate functionality of 2.7 and 51.9 pbw of a uretonimine modified MDI having an NCO value of 31% by weight, an isocyanate functionality of 2.09, a uretonimine content of 17% by weight and a 2,4&#39;-MDI content of 20% by weight. 
     A high pressure, multiple stream dispensing device of KOMET (HP-40/20) was used to make a rigid foam in an open wooden box (50×50×100 cm); blends A-D were fed to the mixing head from separate tanks. The amount of polyisocyanate was such that the isocyanate index was 106. The total shot weight was 3 kg. The rigid foam obtained had a core density of 10.5 kg/m 3  (ISO 845) and a ball rebound of 17% (IS08307). 
     Subsequently foam samples of 10×10×5.5 cm were crushed in the direction of foam rise as follows: the foam was crushed once at 100 mm/min up to 70% CLD followed by 20 crushings at 500 mm/min up to 70% CLD. 
     After crushing a flexible foam was obtained having no major glass-rubber transition between -100° C. and 25° C. and having the properties as given in Table 1. In this example the tank containing the polyisocyanate was kept at 25° C. under 5-8 bar abs using dried air. 
     EXAMPLE 2 
     Example 1 was repeated with the proviso that CO 2  from a pressurized cylinder containing liquid CO 2  was dissolved into the polyisocyanates in the isocyanate-tank until an equilibrium pressure of 6-9 bar abs was established. The polyisocyanate was kept at 25° C. 
     The rigid foam obtained had core density of 10.3 kg/m 3  (ISO 845) and a ball rebound of 19% (ISO8307). 
     After crushing as in example 1 a flexible foam was obtained having no major glass-rubber transition between -100° C. and +25° C. and having the properties given in Table 1. 
     
                       TABLE 1                                                     
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                     example 1                                            
                            example 2                                     
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core density (ISO 845, kg/m.sup.3)                                        
                       12.2     11.2                                      
  ball rebound (ISO 8307, %), measured 54 54                              
  in the direction of crushing                                            
  compression set (ISO 1856, method A, %) 33 33                           
  CLD -25% (ISO 3386/1, kPa) 2.9 2.5                                      
  (CLD = compression load deflection)                                     
  CLD -40% (ISO 3386/1, kPa) 4.4 3.4                                      
  CLD -65% (ISO 3386/1, kPa) 15.7 11.4                                    
  CLD sag factor (ISO 3386/1) (65/25) 5.5 4.6                             
  CLD hysteresis loss (ISO 3386/1, %) 46 46                               
  air flow resistivity (ASTM 3574-86, kPa · s/m.sup.2) 31        
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                                10                                        
 
    
     Compression foam properties were measured in the rise/crushing direction of the foam. 
     DMTA-Test 
     Measurements were carried out according to ISO/DIS 6721-5 on a Rheometric Scientific DMTA apparatus using a 3-point bending mode. Sample test dimensions were: length 1.0 cm, width 1.3 cm, thickness 0.4 cm. Applied strain amplitude 64×10 -4  cm, frequency 1 Hz, heating rate 3° C./min. The foam samples were pre-conditioned at 23° C./50% RH for 24 hours prior testing. The foam samples were quenched to -120° C. (cooling rate 8.5° C./min) and held at that temperature for 5 minutes before heating of the sample was started. 
     EXAMPLE 3 
     Examples 1 and 2 were repeated to prepare a rigid foam with the proviso that 4.5 pbw of water, no phospholene oxide catalyst, 0.75 pbw of DABCO T9 and 0.06 pbw of NaH 2  PO 4  was used and the index was 106. 
     The rigid foams obtained had the following properties: 
     
                       TABLE 2                                                     
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                  no C0.sub.2                                             
                        with CO.sub.2                                     
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core density (ISO 845), kg/m.sup.3                                        
                    12.1    12.0                                          
  ball rebound (ISO 8307), % 26 31                                        
  air flow resistivity 86 6.7                                             
  (ASTM D 3574-86, kPa · s/m.sup.2)                              
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