CA1168800A

CA1168800A - Crosslinker-surfactant for semiflexible energy management polyether polyurethane foams

Info

Publication number: CA1168800A
Application number: CA000384732A
Authority: CA
Inventors: Thomas H. Austin
Original assignee: Individual
Current assignee: Dow Chemical Co
Priority date: 1980-09-29
Filing date: 1981-08-27
Publication date: 1984-06-05
Also published as: DE3136362A1; GB2084595B; ES8704651A1; IT8124190A0; JPS5780420A; FR2491075B1; FR2491075A1; IT1139493B; BR8104219A; GB2084595A

Abstract

CROSSLINKER-SURFACTANT FOR SEMIFLEXIBLE
ENERGY MANAGEMENT POLYETHER POLYURETHANE FOAMS
(D#75,864-F) ABSTRACT OF THE DISCLOSURE

The incorporation of a crosslinker comprising an ethylene oxide adduct of a Mannich condensate of nonyl phenol, diethanolamine and formaldehyde in molded poly-urethane semiflexible foams is disclosed. Semiflexible polyurethane foams incorporating this crosslinker result in foams which offer many advantages including a significant improvement in moldability characteristics and materials utilization as well as other advantages. These improvements are demonstrable over similar formulations using other cross-linkers. The resulting foams find use in energy absorbing applications in the automotive industry and other industries.

Description

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BACKGROUND OF THE INVENTION
Field of the Invention . .
This invention relates to the production of molded polyurethane products.
Description of the Prior Art The particular crosslinker which is found to be advantageous for use in semiflexible energy absorbing foams of this invention has been described as a detergent in U.S.
Patent 2,998,452.
U.S. Patent 4,137,265 discloses the Mannich con-densate precursor of the crosslinker of the present inven-tion. This patent describes and claims a polyol which is a propylene oxide adduct of the Mannich condensate precursor for use in forming rigid polyurethane foams. U.S. Patent 4,137,265 discloses and claims the propylene oxide adduct of nonyl phenol, formaldehyde and diethanolamine. This product is useful as the polyol for rigid foams. It was surprisingly discovered that the ethylene oxide adduc~t of the Mannich condensate found particular advantage in formulations for semiflexible foams.
The automotive industry, in particular, is con-stantly seeking new technologies for increasing the safety of vehicles, while, at the same time, decreasing the weight of the vehicles. A major part of the safety program in the automotive industry is the use of molded polyurethane oams which absorb energy upon impact. The present invention pre-sents an improved method of preparing molded polyurethane foam which in addition to absorbing impact, are lighter in weight resulting in less vehicle weightO

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SUMMARY OF ~IE INVENTION
The invention ls a method for preparing a molded flexible polyure-thane product wherein a polyol, a polyiso cyanate, water and a crosslinker comprising an ethylene oxide adduct of a Mannich condensate prepared from nonyl phenol, diethanolamine and formaldehyde are reacted in a closable mold. The invention is also a molded flexible polyurethane product prepared by the above method.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the molding of polyurethane products, the reaction mixture is charged to a riyid mold which ls suf-ficiently strong that it will not deform when subjected to the pressure within the mold ater the foaming reaction starts. Such a mold can be constructed out of metal, plastic, wood or other materials and combinations of ma terials. In the practice of my invention, the mold should be su~ficiently strong to withstand the molding pressures and also be capable of sealing to the extent that the reaction mixture is not pushed from the mold during the foaming process.
To this preheated mold is charged a sufficient quantity of the foamable polyurethane reaction mi~ture of my invention to overfill the mold by about 10~ to abou-t 20%. The term "overfill the mold" means that guantity of the foamable reaction mixture which is greater than the quantity necessary to just fill the mold after the reaction is complete. I use about 10% to about 20% overfill in the practice of my invention. A greater overfill is possible with very strong and tightly closed molds and would tend to increase the density of the foam.

This overfill and tightly closed mold in con-junction with the reac-tion mixture o~ my invention all combine to pack the polyurethane foam material and give an improved cell structure substantially free of voids and also an improved skin quality of the resulting produc-t. After the material has foamed, it is allowed to stand in the mold for about three to nine minutes usually without any further external heat being necessary to cure the produc-t. O~
course, the foam product can be cwred longer but longer curing is usually unnnessary. The product is removed from the mold after this short cure time and a flexible poly-urethane product is recovered which has a ~ubstantially open cell structure, a good skin and is subs-tantially ~ree of any voids.
In order to successfully practice my invention, it is necessary to use the formulations which I have discovered.
In the one shot process of my invention, the poly-ether polyol component, the organic polyisocyanate, water, crosslinker and catalyst are all brought together simultaneously and allowed to react, foam and cure in the mold without any additional high temperature curing step.
The polyether polyol component (not the cross-linker) useful for the practice of this invention may be a polyol having a functionality of from two to about six and an eguivalent weight of 1600 to about 2000 having about 80 to about 100% primary hydroxyl groups.
The polyether polyols useful in the practice of my invention are prepared by a well known process which involves the reactlon of the polyhydric initiator such as ethylene glycol, pLopylene glycol, low molecular weight polypropylene glycol, -trime-thylolpropane, glycerol, 1,2,6-hexane triol, sorbitol, pentaerythritol, and the like, or mixtures thereof, wi-th a lower alkylene oxide such as propylene oxide and butylene oxide, mixtures thereof, or mixtures of ethylene oxide with propylene oxide and/or butylene oxide. This reaction is carried out in a well known manner with an alkoxylation catalyst, generally an alkali metal hydroxide such as potassium hydroxide. The reaction is continued until the product of an approximate desired molecular weight is obtained. It is necessary to then react the product made as described above with ethylene oxide in order to acquire the desired degree of primary hydroxyl group termination of the polyether chains. This process is described in U.S. Pat.
3,336,242 for example. The percentage of primary hydroxyl groups terminating the polyether chain is generally increased by an addition of ethylene oxide alone; howevex, it will be understood that ethylene oxide mixed with some proportions of propylene oxide will also achieve this result. Also, it is within the scope of my in~ention to use a polyether polyol which may have blocked segments of different alkylene oxides in the molecule and not solely limiting such segments of ethylene oxide to the terminal positions.
~ hile higher functionality polyether polyols may be used, it is preferred to use polyols having molecular weights of 3500 to about 4400 and about 80% or more primary hydroxyl groups which are alkylene oxide adducts of a mixture of a glycol and a triol, for example.
It is especially preferred to use a polyol of about 3900 to 4000 molecular weight which is an ethylene oxide capped propylene oxide adduct of a mixture of a polypropylene 8(~

glycol of about 400 molecular weight and a propylene oxide adduct of glycerol having a mol.ecular weigh-t of about 700. The percentage oE primary hydroxyl groups -terminating this polyol is above abou-t 80%.
The organic polyisocyanate is suitably an organic aroma-tic or aliphatic polyisocyanate such as 3,3'-dichloro-4,4'-biphenyl diisocyanate, diphenyl diisocyanate, ethylene diisocyana-te, propylene-1,2-diisocyanate, 1,4-tetramethylene diisocyana-te, p-phenyle:ne diisocyanate, 2,4- and 2,6-toluene diisocyanates, o,o'-, o,p'- and p,p'-diphenylmethane diisocyana-tes, hexamethylene di.isocyanate, polymethylene polyphenylisocyana-te, and mixtures thereof.
Especially preferred organic polyisocyanates useful in the practice of my invention are those prepared by the phosgenation of the reaction produc-t between aniline and formaldehyde having a functionali-ty oE 2.2 or greater.
While func-tionalities around four and above are possible, they are not read-ily attainable by known processes. It is preferred to use isocyanates having functionalities of about 2.2 to about 3.5 and an especially preferred range is between 2.2 and 2.8. Useful isocyanates are produced by phosgena-ting amine precursors formed in the process described in United States Patents

2,683,730 and 3,362,979, for example.
The foams oL this invention also contain a cross-linker which is the ethylene oxide adduct of the Mannich condensate of nonyl phenol, di-ethanolamine and formaldehyde. Preparation of this Mannich condensate may be found in United States Patent 4,137,265. To this Mannich condensate is added ethylene oxide in an amount to give adducts with hydroxyl number values of 230 to 500 (as meq KOH/g sample). The preferred amount of ethylene oxide yielded an adduct with hydroxyl number value of 470-480 (as meq KOH/g sample).
The polyol component and the organic isocyanate component are mixed in the reaction mixture in such pro-portions that the ratio of isocyanato groups to hydroxyl groups, commonly known as isocyanate index, is from 0.85 to about 1.05 with an especially preferred isocyanate index being about 0.95. The ratio of isocyanato groups to hydroxyl groups includes also any water that is present in the foamable reaction mixture as well as the hydroxyl groups in the crosslinker of the invention.
Water is used to produce the blowing agent and/or adjust the density of the foam in the practice of this in-vention. It is an advantage to use the crosslinker of myinvention so that a higher amount of water may be used and still obtain acceptable foam. It is preferred to use about 2.5 parts by weight of water per 100 parts of polyol com-ponent and it has been found that best results are obtained when the water xange is from about 2.2 to about 4.5 parts by weight per 100 parts of polyol component.
The catalysts useful in my invention includes those normally employed in polyurethane foams including tertiary amines and organometallic compounds. I-t is within the skill of the ordinary worker in the art to choose a workable catalyst system.
E'or example, a partial list of use~ul tertiary amines include trialkylamines (e.g. trimethylamine, tri~
ethylamine), heterocyclic amines~ such as N~alkylmorpholines (e.g., N-methylmorpholine, N-ethylmorpholine, etc.~, l,~

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dimethylpiperazine, triethylenediamine, etc., aliphatic polyamines, such as N,N,N'N'--tetramethyl-l, 3-butanediamine.
Also useful are those catalysts used in the examples which follow. Although the improvement is noted with a variety of catalysts, especially preferred is the two mole propylene oxide adduct of dimethylaminopropylamine.
A partial list of organic tin compounds used as catalysts which are particularly useful in making flexible foams may suitably be a stannous or stannic compound, such as a stannous salt of a carboxylic acid, a trialkyltin oxide, a dialkyltin dihalide, a dialkyltin oxide, etc., wherein the organic groups of the organic portion of the tin compound are hydrocarbon groups containing from 1 to 8 carbon atoms. For example, dibutyltin dilaurate, dibutyltin diacetate, diethyltin diacetate, dihexyltin diacetate, di-2-ethylhexyltin oxide, dioctyltin dioxide, stannous octoate, stannous oleate, etc., or a mixture thereof, may be used.
The catalysts useful in the preparation of poly-ether polyurethane foams described herein based on the combined weight of the hydroxyl-containing compound and polyisocyanate, are employed in an amount of from about 0.05 to about 2.0 weight percent. Preferably, the amount of catalyst used is 0.1-1.5 ~eight percent.
The catalysts used in this invention may be used either alone or in a mixture with one or more other catalysts such as other tertiary amines or with an organic tin compound or other polyurethane catalysts.
C'onventional formulation ingredients may also be employed as needed, such as, for example, foam stabilizers also known as silicone oils or emulsifiers. The foam 8C~

stabilizer may be an organic silane or siloxane. For example, compounds may be used having the formula:
RSi[o-(R SiO)n-(oxyalkylene)mR]3 wherein R is an alkyl group containing from 1 to 4 carbon atoms; n is an integer of from 4 to 8; m is an integer of 20 to 40; and the oxyalkylene groups are derived from propylene oxide and ethylene oxide. See, for example, U.S. Pat. No.

3,194,773.
It is within the scope of the present invention to utilize an extraneously added iner-t blowing agent such as a gas or gas-producing material. For example, halogenated low-boiling hydrocarbons, such as trichloromonofluoromethane and methylene chloride, carbon dioxide, nitrogen, etc., may be used. The inert blowing agent reduces the amount of excess isocyanate and water that is required in preparing flexible urethane foam. Selection of the proper blowing agent is well within the knowledge of those skilled in the art. See for example U.S. Pat. No. 3,072,082.
As the examples which follow demonstrate, it is advantageous to use the present crosslinker which is essen-tially a pentol in conjunction with other low molecular weight materials containing hydroxyl groups having hydroxyl numbers ranging from about 200 to 2000. For example, low molecular weight diols such as ethylene glycol, propylene glycol and dipropylene glycol have been found to be useful as well as carbamate diols and difunctional amine polyols as used in Examples XI-XV which follow. Other similar materials are also useful in conjunction with the crosslinker of the invention.

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In the examples which follow, the invention will be demonstrated and many of the improvements will be apparent from the use of the crosslinker of the invention. ~owever, many of the improvements are of a more subjec-tive nature which are not demonstrable by objective tests which are recognized in the art. I have noticed many improvements and advantages to the use of this crosslinker in semiflexible formulations including:
(1) significant improvement in moldability characteristics, (2) improved material utilization (lower density foam, (3) reduced number of voids in the molded parts,

(4) superior processing with lessened sensitivity to machine conditions,

(5) excellent cell structure and uniform appear-ance,

(6) superior adhesion to vinyl skins used for molded parts,

(7) significantly improved water utilization latitude,

(8) improved cure characteristics (shorter demold time),

(9) lower closed cell content which results in foam which does not shrink, (lO) excellent compatibility of the entire formu-lation which promotes uniform processing.

8C~0 E X A M P L E
.. ..
Preparation of Crosslinker A ~annich condensate was prepared from 1.0 moles nonylphenol, 2.0 moles of diethanolamine, and 2.0 moles of formaldehyde, and water was removed by vacuum stripping.
(See U.S. 4,317,265).
To 61.8 lbs. of the s-tripped Mannich condensate, charged to a 15 gallon kettle and heated to 115C., was added 25.5 pounds of ethylene oxlde at 115-125C., over a period of one hour and twenty minutes. The reaction mixture was digested at 110C for an additional one hour and ten minutes.
Unreacted oxide was removed by vacuum stripping and the product was discharged after cooling. The product viscosity was 21,000 cps, water content was 0.05%, hydroxyl number was 475. This product will be reerred to as Crosslinker A and is the crosslinker of the invention.

E X A M P L E S I I - V
These examples demons-trate use of the crosslinker as the sole crosslinker in formulations designed for semi-flexible instrument panel foam. These cup pours were the initial pours made to determine -the nature and foaming behavior of this material. In these and following examples, the components were weighed into a 5 in. (tall) paper cup, blended with stirring; MONDIJR~) MR was added, the mixture stirred thoroughly and 100-110 g of the mixture was poured into a second 5 in. cup and allowed to foam and rise. Rise time = time in seconds for foam to rise to top of cup. Tack Free - time in seconds for foam surface to freely release when probed with an object such as a tongue depressor or pencil.
15 Formulation, pbw II III IV V
THANOL(3SF-39501 100 . O100 . O100 . O100 . O
Crosslinker A 10.0 8.0 8.0 5.0 Water 2 2.2 2.5 2.5 2.5 THANCAT~DME3 -- 1.0 -- --20 THANCAT~DPA -- -- l.5 1.2 Carbon Black Paste 2.0 2.0 2.0 2.0 A-Comyonent MONDUR MR4 47.0 49.0 49.0 45.5 Cup Data 6 Rise Time, Sec. 260 48 50 75 Tack Free, Sec. 420 150 120 225 Cup Height, inches5 -- 6.8 6.7 6.9 Cup Weight, grams -- 75.8 76.6 71.3 Ethylene oxide capped, propylene oxide adduct of mixed diol and trihydric initiators of about 3950 molecular weight, 2Product of TE~`CO CHEMICAL CO.
3N,N-DimethylethanolamineJ Product of TF.XACO CHEMICAL CO.
4Two mole propylene oxide adduct of dimethylaminopropylamine.
Polymethylene polyphenylisocyanate, Product of Mobay Chem.
Corp.
5Cup Height = Total height of risen foam at final rise height. Cup Weight = Weight of foam in cup after top of foam is cut off even with top rim of cup. This value provides free rise density and the cup height and weight are indicative of the potential foam usage when the foam compound is molded. In later examples, the mixed foam components are poured (Fill Weight, g) into a mold which contains parallel channels and the rising foam is forced to flow around successive 180 turns as it progresses through the channels. That which flows beyond the last channel is trimmed off (part weight, g). This method provides data regarding moldability, foam utilization, foam pressure, potential shrinkage problems (closed cell foam), 6and potential gassing problems.
Foams in Examples II-V exhibited fine uniform cell struc-ture, excellen-t reaction profile. A typical formulation used commercially (see Example X) exhibits rise and tack free times of 80, 330, respectively.

E X A M P L E S V I - X
. . . _ . . _ The examples demonstrate use of the crosslinker o~
the invent~on as an additive for semiflexible foam to promote better cell structure and fu~ction as a surfactant and cell control a~ent.
Formulation, pbw VI VI:[ VIII IX X
B-Component THANOL S~395097.5 97.5 97.5 100.0 97.5 PolyesterlPolyol L-1217 2 2.5 2.5 2.5 __ 2.5 NIAX~50-970 5.8 5.0 5.0 -- 6.0 Crosslinker A 0.2 0.5 1.0 6.0 --Water 3 2.2 2.2 2.2 2.5 2.2 THANCAT~DD 4 0.35 0.35 0.35 0.30 0.35 T~ANCAT~DMDEE0.35 0.35 0.35 0.30 0.35 Carbon Black Paste 2.0 2.0 2.0 2.0 2.0 A-Component MONDUR MR 51.3 50.4 50.4 46.5 45.5 Cup Data Rise Time, Sec. 74 76 80 Tack Free, Sec. 330 330 330 Cup Height, inches 7.3 7.3 7.3 Cup Weight, grams 69.3 69.8 73.0 Molding Data Fill Weight, grams 310 305 305 3025 310 Part Weight, grams 301 288 302 275 306 2Product of WITCO CHEMICAL CO.
3Product of UNION CARBIDE CO., a crosslinker.
2-Dimethylaminoethyl-3-dimethylaminopropyl ether, Product of 4TEXACO CHEMICAI. CO.
5B,B-Dimorpholinodiethyl Ether, Product of TEXACO CHEMICAL CO.
Note superior usage (less weight of foam to fill the mold) when Crosslinker A is substituted direc~].y in a typical commercial formulation (Example X). This superior usage results from utilization of higher water levels which is possible with the use of this crosslinker. Also in these examples, ~hen Crosslinker A is used, improved cell structure resulted.

E X A M P L E S X I - X V
me examples demonstrate use of diluent crosslinkers. 'mese cross-linkers are much lower in functionality than our Crossli~cer A and are sub-stituted into the formulation for part of the Crosslinker A to decrea~se the overall crosslink density and at the same time maintain the required load bearing properties. I~proved u~sage results while inclusion of the Cross-linker A provides the improvem~nts in oe ll structure, moldability and other factors mentioned earlier.
Formulation, pbw XI XII XIII XIV XV

B-Component '~HANOL SF-3950100.0 100.0 100.0 100.0 100.0 Crosslinker A 16.0 6.0 6.0 6.0 6.0 THANOL(~ C-150 2.0 -- -- -- --THANOL ~C-1651 -- 2.0 -~
THANOL ~C-200 2 ~~ ~~ 2.0 -- --THANOL ~TR-380 -- -- -- 2.0 --Dipropylene Glycol -- -- -- - 2.0 Water 2.5 2.5 2.5 2.5 2.5 THANCAT DPA 1.2 1.2 1.2 1.2 1 2 Carbon Black Paste 2~0 2.0 2.0 2.0 2 0 A-Component MONDUR MR 50.2 49.8 49.4 48.2 50.5 Cup Data Rise Time, sec. 60 65 63 67 65 Tack Free, sec.165 195 180 210 195 Cup Height, inches 7.03 7-33 7-33 7-23 7-5 Cup Weight, grams69.5 68.5 72.0 74.0 68.43 Car~amate diol crosslinkers. 150 is 2-hydroxyethyl~2-hydroxyethyl c æbamate, 165 is 2,hydroxyethyl-2-hydroxypropyl carbamate, 200 is 2-hydroxyethyl-2-[2-hydroxyethoxyethyl] carbamate. U.S. 3,595,814 describes 2these products. Products of TEXACO CHEMICAL CO.
Difunctional aminopolyol crosslinker which is the reaction product of ani-line and about 6 to 7 moles of ethylene oxide. See U.S. 4,067,833. Product ~of TEXACO CHEMICAL CO.
~Note cup weight (foam utilization) compared wi-th Examples 3 and 4; improved usage results from incorporation of these diluent crosslinkers. We s-till maintain superior reaction profile and cell structure.

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o E X A M P L E S X V I - X X I I
.. . . ~ ., The examples demonstrate the use o~ diluenk cross-linkers in semiflexible instrumental panel foam. In these examples lower -t~tal crosslinker con-tents are used which provides softer foam than that described in Examples XI - XV.
Formulation, pbw _ XVII XVIII XIX XX XXI XXII

B-Component THANO~ SF-3950 100.0 100.0100.0100.0100.0100.0 100.0 Crosslinker A 3.6 3.63.6 3.0 2.4 2.5 2.0 Dipropylene glycol 2.4 -- -- --Propylene glycol -- 2.4 2.4 2.0 1.6 2.5 2.0 Water 2.5 2.52.5 2.5 2.5 2.5 2.5 THANCAT~DPA 1.0 1.01.6 1.6 1.6 1.6 1.6 Carbon Black Paste2.02.0 2.0 2.0 2.0 2.0 2.0 A-Component MONDUR MR 48.3 51.651.8 49.9 48.0 51.0 48.9 Cup Data Rise Time~ sec. 92 82 60 63 66 60 64 Tack Free, sec.l 270270 165 180 210 180 195 Cup Height, inl 7.47.5 7.7 7.7 7.8 7.8 7.9 Cup Weight, g. 68.666.0 63.6 63.2 63.6 61.5 62.1 Nolding Data Fill Weight 307 305313 307 306 312 308 Part Weight 301 399308 300 296 294 308 lWe obtain superior foam utilization when diluent crosslinkers are used, yet we do not sacrifice any of the benefits offered by Crosslinker A when its concentration is reduced.

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E X A M_P L E S X X I I I - X X I ~
These exampLes demonstra-te ln~luence of higher water levels in the formulations. Hlgher water levels give lower denslty foam, better foam usage, less weight per part.
Hlgher ~7ater levels in a ~ormulation usually lead to poorer moldability, increased voids, ~ut the use of Crossllnker A
allows these higher levels to be used.
Formulation, pbw ~YXIII XXIV XXV XXVI XXVI:[ XXVIII XXIX

B-Component THANOL SF-3950100.0100.0100.0 100.0 100.0 100.0 100.0 Crosslinker A3.0 3.0 3.0 3.0 3.0 3.0 6.0 Propylene Glycol 3.0 3.0 3.0 3.0 3.0 3.0 --Water 2.5 2.6 2.7 2.8 2.9 3.0 3.0 TH~NCAT DPA1.2 1.2 1.2 1.2 1.2 1.2 1.2 Carbon Black Paste2.0 2,0 2.0 2.0 2.0 2.0 2.0 A-Component MOND~R MR 53.1 54.4 55.3 51.1 58.8 59,8 54.3 Cup Data Rise Time, sec. 76 75 74 75 68 65 60 Tack Free, sec.l 240 240 240 240 240 210 240 Cup Height, in 7.7 7.9 7.9 7.9 8.0 8.3 8.1 Cup Weight, g,l 63.7 62.3 60.9 60.1 59.9 54.8 58.1 ~ Molding Data ; Mold Fill Wt.l 318 322 323 322 324 330 --Mold Part Wt.298 286 28`4 286 280 263 --Obvious benefits result from higher water levels. (The normal range in commercial foLmulations is 1.8 to 2.2.) Later examples will show that levels above the standard 2.2 yield good moldability systems.

E X A M P L E S X X X X X X X V I
The examples demons-trate the use of formulations (described in previous examples in laboratory hand mix studies) in machine pours, wherein cups, molds and instrument panel pads are poured. Although cup pours and mold pours are used to indicate moldability, potential reduction of void.s, foam usage and higher water utility; machine poured pads fully illustrate these things in actual end use.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows;

1. In a method for preparing a molded flexible polyurethane product wherein a polyol, a polyisocyanate, water and a crosslinker are reacted in a closable mold to form the polyurethane product the improvement which comprises using a crosslinker comprising an ethylene oxide adduct of a Mannich condensate prepared from the reaction of nonyl phenol, diethanolamine and formaldehyde.

2. A method as in Claim 1 wherein the crosslinker has a hydroxyl number ranging from about 230 to about 500.

3. A method as in Claim 1 wherein the crosslinker has a hydroxyl number ranging from about 470 to 480.

4. A method as in Claim 1 wherein the polyol is an alkylene oxide adduct of an initiator having both diol and triol components.

5. A method as in Claim 3 wherein the polyol is terminated by at least 80% primary hydroxyl groups and has an equivalent weight ranging from about 1600 to 2000.

6. A method as in Claim 1 wherein the polyisocyanate is a polymethylene polyphenyl polyisocyanate.

7. A method for preparing a molded flexible polyurethane product comprising reacting in a closable mold a polyol, a poly-isocyanate, water and a crosslinker which comprises an ethylene oxide adduct of a Mannich condensate prepared from the reaction of nonyl phenol, diethanolamine and formalde-hyde.

8. A method as in Claim 6 wherein the crosslinker has a hydroxyl number ranging from about 230 to about 500.

9. A method as in Claim 6 wherein the crosslinker has a hydroxyl number ranging from about 470 to 480.

10. A method as in Claim 8 wherein the polyol is an alkylene oxide adduct of an initiator having both diol and triol components.

11. A method as in Claim 6 wherein the polyol is terminated by at least 80% primary hydroxyl groups and has an equivalent weight ranging from about 1600 to 2000.

12. A method as in Claim 1 wherein a low molecular weight diol is also present.

13. A method for preparing a molded flexible poly-urethane product comprising reacting in a closable mold a polyol which is alkylene oxide adduct of an initiator having both diol and triol components said polyol having a primary hydroxyl con-tent of at least 80%, a polymethylene polyphenyl polyisocyanate, water and a crosslinker which comprises an ethylene oxide adduct of a Mannich condensate prepared from nonyl phenol, diethanolamine and formaldehyde said crosslinker having a hydroxyl number ranging from about 230 to 500.

14. A method as in Claim 13 wherein a low molecular weight diol is also present.

15. A method as in Claim 13 wherein a catalyst is present comprising two mole! propylene oxide adduct of dimethylaminopropylamine.