MXPA96000199A - Flexi open cellular polyurethane foam - Google Patents

Abstract

A flexible open cell polyurethane is disclosed which is the reaction product of a mixture comprising (a) a polyisocyanate component comprising monomeric or polymeric MDI, and (b) a polyol component having at least 50 percent its OH functional groups as secondary OH groups

Description

"FLEXIBLE OPEN CELL POLYURETHANE FOAM" INVENTORS: DUANE A. HEYMAN, THEODORE M. SMIECINSKI and DONALD C MINENTE American, domiciled in 1902 South Raisinville Monroe, Michigan 48161; 16268 Truwood, Oodhaven, Michigan 48183 and 8789 Par, Grosse Lie, Michigan 48138 United States of America all rights to BASF CORPORATION, a company duly organized and incorporated in accordance with the Laws of the State of Delaware, United States of America , with address at 1419 Biddle Avenue, Wyandotte, Michigan 48192-3736 United States of America, by the invention that is described below.

FIELD OF THE INVENTION This invention relates to polyurethane foams in general and to processes for the preparation thereof. Specifically, the invention relates to flexible open-cell foams such as those useful for underlying carpet or furniture layer applications.

BACKGROUND OF THE INVENTION Open cell flexible polyurethane foams are widely used in a variety of applications. They offer high resilience and high load-carrying properties, making them useful for applications such as furniture cushions and underlay layer. Open cell flexible polyurethane foams have traditionally been prepared from TDI components polyisocyanate (toluene diisocyanate). However, for many higher load carrying applications, such as the underlying carpet layer, TDI-based foams typically have a density of < 48 grams per cubic centimeter and are subject to fatigue when the load carrying capacity is reduced. This is particularly acute for the underlying layer of the mat, where high traffic areas may suffer from premature fatigue of the underlying carpet layer. MDI has been used to make high density open cell flexible polyurethane foams. These foams can provide dense foams (of> 48 grams per cubic centimeter), which may offer advantages in fatigue resistance compared to TDI-based foams. TDI based foams can not be easily prepared with densities greater than 48 grams per cubic centimeter. MDI-based foams can also offer good tensile strength (> 2.46 kilograms per square centimeter) and tear strength (> 49.11 kilograms per meter). However, the increased reactivity of the polyisocyanate components based on MDI can cause different problems. Depending on the nature of the polyol used in combination with the MID-based polyisocyanate, the foam produced can exhibit great hardness rendering it unsuitable for cushioning use. Also, the necessary degree of open cell structure in the foam may not be achieved. A significant problem encountered in the preparation of MDI based open cell flexible polyurethane foams has been the formation of so-called "cream balls" in the foam preparation equipment. The cream balls are regions of polyurethane foam that are formed and remain in the foam preparation equipment, causing fouling and reducing the performance of good quality foam from the equipment. This problem is particularly serious in foam distributing machines of trough-type slab material. It is towards the problem of providing open cell flexible polyurethane foams that exhibit good fatigue properties and reduced tendency to fouling of the foam dispensing equipment to which the present invention is directed.

SUMMARY OF THE INVENTION In accordance with the present invention, a flexible open cell polyurethane foam is provided which is the reaction product of a mixture comprising (a) a polyisocyanate component consisting of monomeric or polymeric MDI, and (b) a component of polyol having at least 50 percent of its OH functional groups as secondary OH groups. The foams according to the present invention provide good fatigue properties and reduced tendency to fouling of the foam dispensing equipment.

DESCRIPTION OF THE PREFERRED MODALITIES The polyisocyanate component (a) comprises monomeric and polymeric MDI. These are well known in the art, and include 4,4'-, 2,4'-, and 2,2'-diphenylmethane diisocyanate, various polyphenylenepolyethylene polyisocyanates (polymeric MDI), and mixtures of some or all of these compounds The polyisocyanate component (a) may also include one or more other aliphatic, cycloaliphatic, araliphatic, and / or aromatic polyisocyanates. Specific examples of these other polyisocyanates include: alkylene diisocyanates having from 4 to 12 carbon atoms in the alkylene radical such as 1,12-dodecane diisocyanate, 2-ethyl-1,4-tetramethylene diisocyanate, diisocyanate 2 -methyl-l, 5-pentamethylene, 1,4-tetramethylene diisocyanate, and 1,6-hexamethylene diisocyanate; cycloaliphatic diisocyanate such as 1,3- and 1,4-cyclohexane diisocyanate as well as any of the mixtures of these isomers, 3-3,5-trimethyl-5-isocyanatomethylcyclohexane-1-isocyanate (isophorone diisocyanate), and diisocyanate of 2,4- and 2,6-hexahydrotoluene, as well as the corresponding isomeric mixtures; 4,4'-2,2'-, and 2,4'-dicyclohexylmethane diisocyanate as well as the corresponding isomeric mixtures; and other aromatic polyisocyanates such as 2,4- and 2,6-toluene diisocyanate (TDI) and the corresponding isomer mixtures. The polyisocyanate component (a) must contain at least 70 percent by weight of MDI, and preferably from 75 percent to 90 percent by weight of MDI. In a preferred embodiment, the polyisocyanate component (a) comprises a mixture of monomeric and / or polymeric MDI plus TDI.

Any of the aforementioned polyisocyanates can be used individually or in the form of mixtures. Frequently, so-called modified multivalent isocyanates are used, that is, products obtained by the partial chemical reaction of the organic diisocyanates and / or polyisocyanates. Examples include diisocyanates and / or polyisocyanates containing ester groups, urea groups, biuret groups, allophanate groups, carbodiimide groups, isocyanurate groups and / or urethane groups. Specific examples include organic polyisocyanates, preferably aromatics containing urethane groups (also known as isocyanate prepolymers) and having a free NCO content of 20 percent to 46 percent by weight, preferably 25 percent to 40 percent by weight. weight percent, based on total weight, which can be prepared by reacting the polyisocyanate with diols, triols, dialkylene glycols, trialkylene glycols or low molecular weight polyoxyalkylene glycols, with a molecular weight up to 1500. Examples of polyols useful for preparing the prepolymers of isocyanate include diethylene glycol, dipropylene glycol, polyoxyethylene glycol, polyoxypropylene glycol, polyoxyethylene glycol, polyoxypropylene glycol and polyoxypropylene polyoxyethylene glycols or triols. The isocyanate prepolymers may optionally be mixed together or mixed with unmodified organic polyisocyanates such as 2,4'- and 4,4'-diphenylmethane diisocyanate, polymeric MDI, 2,4- and / or 2,6-toluene diisocyanate. The crude polyisocyanates can also be used in the compositions of the present invention, such as crude toluene diisocyanate obtained by the phosgenation of a mixture of diamines of toluene or crude diphenylmethane diisocyanate obtained by the phosgenation of crude diphenylmethane diamine. The polyol component (b) comprises one or more polyol compounds in such a way that at least 50 percent of the OH functional groups of the polyol component are secondary OH groups. Representative polyols that can be employed in the invention are well known to those skilled in the art. Representative polyols include polyesters containing polyhydroxyl, polyoxyalkylene polyether polyols, polyhydric polyurethane polymers, polyhydroxy containing phosphorus compounds, and alkylene oxide adducts of the polyhydric polythioethers, polyacetals, polyols and aliphatic thiols, ammonia and amines including the aromatic, aliphatic and heterocyclic amines, as well as mixtures thereof. The alkylene oxide adducts of the compounds containing two or more different groups (e.g., amino-alcohols) within the classes defined above may also be used. In addition, the alkylene oxide adducts of compounds containing a group of SH and an OH group as well as those containing an amino group and a group of SH can be used. In general, the equivalent weight of the polyols will vary from 500 to 10,000, preferably from 750 to 3000. Any suitable hydroxy-terminated polyester such as that which can be prepared for example from polycarboxylic acids and polyhydric alcohols can be used. Any suitable polycarboxylic acid such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, glutaconic acid, terephthalic acid, and the like can be used. Any suitable polyhydric alcohol including both aliphatic and aromatic may be used, for example, ethylene glycol, propylene glycol, trimethylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,4-pentanediol. , 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, glycerol, 1,1-trimethylolpropane, 1,1-trimethylolethane, 1,2,6-hexanetriol, alpha-methylglucoside, pentaerythritol and sorbitol. Included within the term "polyhydric alcohol" are phenol-derived compounds such as 2,2-bis (4-hydroxyphenyl) propane, which is commonly referred to as Bisphenol A. In order to obtain the secondary hydroxy functional groups, the polyester must to undergo terminal inactivation with a secondary hydroxyl-containing polyol, such as 1,2-propanediol, 1,3-butanediol, 1,2-butanediol, or similar materials. Also, polyols containing ester groups can be used in the present invention. These polyols are prepared by the reaction of an alkylene oxide with an organic dicarboxylic acid anhydride and a compound containing reactive hydrogen atoms. A more comprehensive discussion of these polyols and their method of preparation can be found in US Patent Numbers 3,585,185; 3,639,541 and 3,639,542. While a variety of polyol compounds can be used to incorporate the secondary hydroxyl groups, such as the polyester polyols described above, the polyol component is preferably composed primarily of polyether polyol (s). The polyether polyol composition useful in the practice of the invention contains a predominant amount of secondary hydroxyl groups, with a composition consisting of all secondary hydroxyl groups being preferred. By a predominant amount of the polyether polyol composition containing the secondary hydroxyl group is meant that at least 50 weight percent of the hydroxyl groups should be secondary hydroxyl groups such as those derived from the oxide of propylene. It may be preferable to add the ethylene oxide during the chain lengthening of the polyether polyol to prepare a polyether block polyether or internal polyether as long as less than 50 weight percent of the polyol is terminated with the primary hydroxyl groups such as those derived from ethylene oxide. While it is within the scope of the invention to add the above-mentioned small amounts of ethylene oxide to an initiator molecule as an inactivation, it is preferred to prepare a polyoxyalkylene polyether polyol exclusively containing secondary hydroxyl groups. Methods for producing polyether polyols are well known and include those polyethers prepared from the catalyzed addition based on an alkylene oxide such as ethylene oxide, propylene oxide or butylene oxide, preferably ethylene oxide, to the initiator molecule which contains, on average, two or more active hydrogens. Polyoxyalkylene polyether polyols are well known in the art and can be prepared by any known process, such as for example the process disclosed by Wurtz in 1859 and the Encyclopedia of Chemical Technology, Volume 7, pages 257 to 262, published by Interscience Publishers, Inc. (1951). In a preferred embodiment of the invention, the polyether polyol comprises 5 percent to 20 weight percent ethylene oxide units. Also, at least 50 percent, and preferably 70 percent to 100 percent of the hydroxyl functional groups in the polyol, are preferably secondary hydroxyl groups, preferably derived from propylene oxide inactivation groups. in polyether polyols. Examples of initiator molecules are diethylene glycol, ethylene glycol, dipropylene glycol, propylene glycol, trimethylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,4-pentanediol, 1,5-pentanediol, 1,6- hexanediol, 1,7-heptanediol, glycerin, 1,1-trimethylolpropane, 1,1-trimethylolethane, 1,2,6-hexanetriol or triethylolpropane. Particularly preferred initiators include trimethylolpropane, glycerin, propylene glycol, and mixtures of polyoxyalkylene polyether polyols initiated in this manner, with glycerin, and trimethylolpropane being especially preferred. Suitable alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, amylene oxide, and mixtures of these oxides. The reaction product of ethylene oxide or a mixture of ethylene oxide and propylene oxide with one of the aforementioned initiators is preferred followed by inactivation of the polyether with propylene oxide, to yield a polyether polyol having only hydroxyl groups predominantly. In one embodiment, the polyether polyol has an equivalent weight of 1000 to 2000, an average functionality of about 2.0 to 4, and a hydroxyl number of 20 to 60. The polyol component (b) may also contain solid polymer particles . Polyols containing preferred polymer particles are so-called graft polyols comprising a carrier polyol containing predominantly secondary hydroxyl groups together with the polymer particles. Graft polyols are well known in the art and are typically prepared by the in situ polymerization of one or more vinyl monomers, preferably acrylonitrile and styrene, in the presence of a polyether or polyester polyol, particularly polyols containing a small amount of natural or induced unsaturation followed by optional mixing with additional liquid polyol. Methods for preparing these graft polyols can be found in columns 1 to 5 and in the Examples of US Pat. No. 3,652,639; in columns 1 to 6 and in the Examples of the North American Patent Number 3,823,201; particularly in columns 2 to 8 and the Examples are U.S. Patent Number 4,690,956; and in US Pat. Nos. 4,524,157, 4,661,531, Re 33,291, all of which are incorporated herein by reference. In one embodiment of the invention, the polyol component (a) has a solids content of 4 percent to 60 percent by weight, and the polymer particles are acrylonitrile-styrene with an acrylonitrile: styrene ratio of (4: 1) to 1: 4). The polyurethane foams employed in the present invention is generally prepared by reacting a polyoxyalkylene polyether polyol with an organic polyisocyanate in the presence of a swelling agent and optionally in the presence of additional polyhydroxyl-containing components, chain elongation agents. , catalysts, surfactants, stabilizers, colorants, fillers or fillers and pigments. Chain elongation agents can also be used in the preparation of the polyurethane foams according to the present invention. These include compounds having at least two functional groups that carry active hydrogen atoms such as water, hydrazine, primary and secondary diamines, aminoalcohols, amino acids, hydroxy acids, glycols or mixtures thereof. A preferred group of chain elongation agents includes water, ethylene glycol, 1,4-butanediol and primary and secondary diamines which react more readily with the prepolymer than water, such as phenylenediamine, 1-cyclohexane-bis- (methylamine), ethylenediamine, diethylenetriamine, N- (2-hydroxypropyl) ethylenediamine, N, N'-di (2-hydroxypropyl) ethylenediamine, piperazine and 2-methylpiperazine. Any suitable catalyst can be used in the preparation of foams according to the invention, including tertiary amines such as, for example, triethylenediamine, N-methylmorpholine, N-ethylmorpholine, diethylethanolamine, l-methyl-4-dimethylaminoethylpiperazine, 3-methoxypropyl dimethylamine, propylene diamine. N, N, N'-trimethylisopropyl, 3-diethylaminopropyldiethylamine, dimethylbenzylamine, and the like. Other suitable catalysts are, for example, stannous chloride, dibutyltin di-2-ethylhexanoate, stannous oxide, as well as other organometallic compounds such as those disclosed in U.S. Patent No. 2,846,408.

A surfactant is often useful for the production of high quality polyurethane foam according to the present invention to prevent foam crushing and to activate a good cell structure. Numerous surfactants have been found to be satisfactory. Nonionic surfactants such as silicone-based compounds are preferred. Other useful surfactants include polyethylene glycol ethers of long chain alcohols, tertiary amine or alkanolamine salts of long chain alkyl acid sulfate esters, alkylsulphonic esters, and alkylarylsulfonic acids. Appropriate processes for the preparation of cellular polyurethane foams are disclosed in U.S. Reissue No. 24,514 together with the appropriate machinery for use therewith. When water is added to generate CO2 as the swelling agent, corresponding amounts of an excess of isocyanate can be used to react with the water. It is possible to continue the preparation of polyurethane foams by a prepolymer technique wherein an excess of the organic polyisocyanate is reacted in a first step with the polyol of the present invention, in order to prepare a prepolymer having free isocyanate groups that then they are reacted in a second step with water and / or additional polyol- to prepare a foam. Alternatively, the components can be reacted in a single work step commonly known as a "one-shot" technique for preparing polyurethanes. In addition, instead of the water, low boiling point hydrocarbons such as cyclopentane, pentane, hexane, heptane, pentene and heptene can be used as the swelling agents; azo compounds such as azohexahydrobenzodinitrile; halogenated hydrocarbons CFCs such as dichlorodifluoromethane, trichlorofluoromethane, dichlorodifluoroethane, HCFC hydrocarbons, HFC hydrocarbons and ethylene chloride. The composition of the present invention is particularly useful for producing foams in the form of a slab or bun. These foams can be prepared: (a) by mixing (1) a polyisocyanate component comprising monomeric or polymeric MDI, and (2) a polyol component having at least 50 percent of its OH functional groups as secondary OH groups to form a polyurethane reaction mixture, (b) applying the polyurethane reaction mixture to a movable substrate to form a polyurethane bun, and (c) removing the polyurethane bun from the substrate. The problem of cream ball formation can be particularly acute in so-called trough foam forming machines. Trough machines are well known in the art and are described for example in U.S. Patents 4,074,960 and 4,298,557. Therefore, in the preferred embodiment of the invention, the above-described process further comprises the step of filling a distributor tundish having an overflow weir with the polyurethane reaction mixture so that the polyurethane reaction mixture flows through the polyurethane reaction mixture. from overflow weir to the movable substrate. The invention is further described by the following examples.

Example 1 Formulations (A), (B), (C) and (D) were weighed into a paper cup with a capacity of .946 liter, mixed for 20 seconds at 1500 revolutions per minute using a small German type mixing paddle. , then positioned so that spindle number 4 of a Brookfield Digital Viscometer was properly immersed in the mixed components to record the viscosities until the mixture gelled (> 200,000 centipoise).

Formulation PB A B C D Polyol A '100.0 - - 100.0 Polyol B '- 100.0 - Polyol C - - 100. Dibutyltin dilaurate 0.02 0.02 0.02 0.02 Isocyanate A 5.5 5.5 5.5 TDI (80/20) - - - 3.9 Time to gelation, Minutes 3 9.5 13.5 < fifteen Polyol A 'is a graft polyol having 31 percent solids of acrylonitrile: styrene in polyol A carrier. Polyol A is a polyether polyol of PO initiated with TMP-having an EO inactivation for a total EO content of 13 percent and an OH number of 35. Polyol B * is a graft polyol having 31 percent of acrylonitrile: styrene solids in the carrier polyol which is a mixture of 38:62 of Polyol A and B. Polyol B is a polyether polyol of EO / PO heteric initiated with glycerin having an activation of PO and a OH number of 35. Polyol C is a graft polyol that has 31 percent solids of acrylonitrile: styrene in the carrier Polyol B. Isocyanate A is a mixture of 77:23 MDI / TDI modified with carbodiimide.

Example 2 The manually mixed foams were then prepared according to the following procedure. All of the foam-forming components, except the isocyanate, were weighed into a 1,893 liter paper cup and blended for 30 seconds at 1800 revolutions per minute with a large German-type mixing paddle. The isocyanate was weighed in a separate container, added to the above described mixture and mixed for 10 seconds at 1800 revolutions per minute. This foam-forming mixture was immediately emptied into a plastic device with a capacity of 18,925 liters and observations were recorded. The manually-mixed foam-forming reactivity of formulation F was significantly slower than formulation E based on the observed times of cream formation and expansion. The physical properties between the foams prepared from these two formulations were similar. The formulations and physical properties are listed in Table I. The reactivity of the hand-mixed foam formulation of formulation H was significantly slower than the formulation G based on the times observed for cream formation and expansion. The physical and fatigue properties were similar. The formulations and physical properties are listed in Table II.

Example 3 A laboratory-scale slab material machine was used using a tundish to simulate the production environment under which fouling and accumulation could be observed. All the components were supplied in a regulated manner in separate streams to a pouring head type of pin mixer which then distributes the foam forming mixture to the tundish. The foam-forming reaction mass was allowed to foam towards the movable conveyor belt. This continuous foaming operation was allowed to run until a foam block of 9,144 meters was produced. The observations made while the foam was emptied with formulation I showed fouling (cream balls) and accumulation in the tundish. The observations made while the foam was emptied with formulation J showed some soiling and accumulation in the tundish that started at about 2.5 minutes towards emptying and was comparable with the fouling of formulation I when the emptying stopped after 4 minutes . The observations made while the foam was emptied with the K formation did not show fouling or accumulation that would have occurred in the tundish for a void that lasted approximately 6 minutes. The physical and fatigue properties were similar when comparing the foam of formulation I with the foams of formulations J and K. The formulations and physical properties are listed in Table III. As used herein, catastrophic fatigue is defined as the castor oil fatigue test of Method 3574-91 of the American Society for the Testing of Materials that was modified since no carpet or mat was used and the number of cycles is 40,000.

TABLE I Hand-Mixed Foam Formation Study Formulations E F Polyol A '100.0 -Polol B' - 100.0 Silicone Surfactant 1.6 1.2 Diethanolamine 0.7 0.7 Flame Retardant 3.0 3.0 33 percent of triethylene diamine in dipropylene glycol 0.15 0.15 dibutyltin dilaurate 0.1 0.1 Water 2.0 2.0 Isocyanate A 40.2 40.2 Observations of the mixture by hand Time to form the cream, seconds 25 29 Expansion time, seconds 109 126 Height, centimeters 26.67 27. 94 Comments HB, SB HB HB = Sanitary Bubbles, SB = Posterior sigh Properties of the Foam Density, gram / cubic cm 57.12 51.2 Stress, kilograms per square cm 3.16 2.88 Voltage, H.A. kilograms per square cm 3.23 2.60 Elongation,% 90 110 Break, kilogram / meter 62.50 53.57 Resilience,% 37 29 IFD, .454 kilograms / 3,225 square meters (10.16 centimeters) 25% 138.1 121.7 65% 351.0 257.4 % Return 92.2 83.5 Balancing factor 2.54 2.12 % Recovery 92.2 83.5 Compression Solidification,% Solidification 75% 15 16 Air Flow, cubic centimeters / minute .849 .283 TABLE II Evaluation of Hand Mixed Foam Formation Formulations G H Polyol A '100.0 - Polyol C - 100.0 silicone surfactant 1.6 1.2 Diethanolamine 0.7 0.7 flame retardant 3.0 3.0 33 percent triethylenediamine in dipropylene glycol 0.15 0.15 Dibutyltin dilaurate 0.1 0.1 Water 2.0 2.0 Isocyanate A 40.2 40.2 Observations of the Mix by Hand Time to form the cream, seconds 24-26 27-29 Expansion time, seconds 88 107 Height, centimeters 26.67 27.94 Comments HB, SB HB, SB Properties of the Foam Density, gram / cubic cm 48 50.88 Stress, kilograms per square centimeter 2.53 2.81 Stress, H.A. kilograms per square centimeter 2.32 2.81 Elongation,% 103 137 Break, kilogram / meter 62.50 71.43 Resilience,% 32 24 IFD, .454 kilograms / 3,225 square meters (10.16 centimeters) 25% 84.8 74.7 65% 208.6 177.0 25% Return 56.6 49.7 Camber factor 2.46 2.37 Recovery% 66.2 66.6 Solidifications by Compression,% of Solidification 75% 10 16 Air Flow, cubic centimeter / minute .566 .283 Catastrophic Fatigue, 40K cycles, No Mat or Height Mat,% Loss 1.9 3.0 65% IFD,% Loss 72.5 73.2 TABLE III Results of Laboratory Emptyings Formulation I_ J K Polyol A '100.0 Polyol B' - 100.0 Polyol C - 100.0 Silicone Surfactant 1.6 1.6 1.3 Diethanolamine 0.7 0.7 0.7 Flame retardant agent 3.0 3.0 3.0 33 percent triethylene diamine in dipropylene glycol 0.2 0.2 0.2 dibutyltin dilaurate 0. 1 0. 1 0. 1 Water 2. 0 2. 0 2. 0 Isocyanate A, index 110 110 110 Properties of Foam Density, gram / cubic cm 53.28 59.68 52.48 Stress, kilograms per square cm 3.94 4.01 3.80 Stress, H.A. kilograms per square centimeter 4.08 4.43 4.29 Elongation,% 100 77 100 Break, kilogram / meter 51.79 55.36 66. 07 Resilience,% 36 27 28 IFD, .454 kilogram / 3,225 square meter (10.16 cm) 25% 161 176 161 65% 429 484 413 % Return 97 112 99 Comba Factor 2.67 2.75 2.56% Recovery 60 47 61 Solidifications by Compression,% solidification (75%) 17 39 20 Aged in Humid for 3 hours at 105 ° C CLD,% of Original, 50% 95 91 96 Compression,% of Solidification 75% 19 35 19 Air Flow, cubic centimeter / minute 0.4 0.1 0.1 Catastrophic, 40K cycles - Without Rug or Carpet Height,% Loss 4.5 4.0 3.1 65% IFD,% Withheld 70 72 66

Claims (23)

N O V E D A D I N V E N C L I N Having described the invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property:

1. A flexible open cell polyurethane foam which is the reaction product of a mixture comprising (a) a polyisocyanate component comprising monomeric or polymeric MDI, and (b) a polyol component having at least 50 percent its OH functional groups as secondary OH groups.

2. A flexible open cell polyurethane foam according to claim 1, wherein the polyol component has an equivalent weight of 700 to 2500.

3. A flexible open cell polyurethane foam according to claim 2, in wherein the polyol component has an equivalent weight of 1000 to 2000.

4. A flexible open cell polyurethane foam according to claim 2, wherein the polyol component comprises a dispersion of dispersed polymer particles in a carrier polyol.

A flexible open cell polyurethane foam according to claim 1, wherein the polyol component comprises a dispersion of dispersed polymer particles in a carrier polyol.

6. A flexible open cell polyurethane foam according to claim 5, wherein the polymer particles are a copolymer of acrylonitrile and styrene.

7. A flexible open cell polyurethane foam according to claim 6, wherein the ratio of acrylonitrile to styrene is between 4: 1 and 1: 4.

8. A flexible open cell polyurethane foam according to claim 1, wherein the polyol component has an average hydroxyl functionality of 2 to 4.

9. A flexible open cell polyurethane foam according to claim 6. , wherein the polyol component has an OH number of 20 to 60.

10. A flexible open cell polyurethane foam according to claim 1, wherein the polyol component comprises a polyether polyol having an equivalent weight of 1000 to 2000.

11. A flexible open cell polyurethane foam according to claim 9, wherein the polyether polyol comprises from 5 percent to 20 percent of ethylene oxide units and is at least partially inactivated with propylene oxide.

12. A flexible open cell polyurethane foam according to claim 1, wherein the polyol component has from 70 percent to 100 percent of its OH functional groups as secondary OH groups.

13. A flexible open cell polyurethane foam according to claim 12, wherein the polyol component is a polyether polyol and the secondary OH groups are derived from inactivation groups of propylene oxide.

14. A flexible open cell polyurethane foam according to claim 1, wherein 100 percent of the OH functional groups of the polyol component are secondary OH groups.

15. A flexible open cell polyurethane foam according to claim 14, wherein the polyol component is a polyether polyol and the secondary OH groups are derived from inactivation groups of propylene oxide.

16. A flexible open cell polyurethane foam according to claim 1, wherein the polyisocyanate component further comprises TDI.

17. A flexible open cell polyurethane foam according to claim 1, having a density of at least 48 grams per cubic centimeter.

18. A flexible open cell polyurethane foam according to claim 16, which has a catastrophic fatigue of at least 65 percent which is retained at 65 percent DFI.

19. A flexible open cell polyurethane foam according to claim 1, which has a catastrophic fatigue of at least 65 percent withheld to 65 percent of DFI.

20. A flexible open cell polyurethane foam according to claim 1, having a tensile strength of at least 2.46 kilograms per square centimeter.

21. A flexible open cell polyurethane foam according to claim 1, having a breaking strength of at least 49.11 kilograms per meter.

22. A method for preparing a flexible open cell polyurethane foam comprising the steps of (a) mixing (1) a polyisocyanate component comprising monomeric or polymeric MDI, and (2) a polyol component having at least 50 percent by weight. one hundred of its OH functional groups as secondary OH groups to form a polyurethane reaction mixture, (b) applying the polyurethane reaction mixture to a movable substrate to form a polyurethane bun, and (c) remove the polyurethane bun from the substrate. A method according to claim 21 further comprising the step of filling a distributor tundish having an overflow weir with the polyurethane reaction mixture so that the polyurethane reaction mixture flows through the overflow weir towards the movable substrate. "FLEXIBLE OPEN CELL POLYURETHANE FOAM." SUMMARY OF THE INVENTION A flexible open cell polyurethane is described which is the reaction product of a mixture comprising (a) a polyisocyanate component comprising monomeric or polymeric MDI, and (b) a polyol component having at least 50 percent its OH functional groups as secondary OH groups. In testimony of which, I have signed the previous description and novelty of the invention as agent of BASF CORPORATION, in Mexico City, Federal District today, January 11, 1996. EDUARDO CORREA E.