JP3671564B2 - Method for producing flexible polyurethane foam - Google Patents

Description

表１において、
ポリイソシアネート（１）：異性体１８％含有ＭＤＩ
ポリイソシアネート（２）：多核体６０％、ＭＤＩ４０％、ＭＤＩ中の異性体１５％
ポリイソシアネート（３）：ＴＤＩ（コロネートＴ−８０、日本ポリウレタン工業製 ）
ポリオール（１）：分子量８０００、官能基数４、ポリエーテルポリオール
ポリオール（２）：分子量２０００、官能基数２、ポリエーテルポリオール
ポリオール（３）：分子量１０００、官能基数２、ポリエーテルポリオール
ポリオール（４）：分子量２５５０、官能基数２、ポリエステルポリオール
【００２２】
［ポリオール予備組成物の調製］
製造例３
ポリオール（６）、（９）及び（１０）、触媒（１）及び（３）、水をこの順で加えて混合し、ポリオール予備組成物（イ）を得た。
原料組成を表２にまとめて示す。
【００２３】
製造例４
ポリオール（５）、（６）、（７）及び（８）、触媒（１）及び（２）、水をこの順で加えて混合し、ポリオール予備組成物（ロ）を得た。
原料組成を表２にまとめて示す。
【００２４】
【表２】

表２において、
ポリオール（５）：分子量７０００、官能基数３、ポリエーテルポリオール
ポリオール（６）：分子量７０００、官能基数４、ポリエーテルポリオール
ポリオール（７）：アクリルニトリルとスチレンからなるポリマーポリオール
ポリオール（８）：分子量１５００、官能基数３、ポリエーテルポリオール
ポリオール（９）：分子量６０００、官能基数３、ポリエーテルポリオール
ポリオール（１０）：アクリルニトリルからなるポリマーポリオール
触媒（１）：ＴＥＤＡ Ｌ−３３、東ソー製
触媒（２）：Ｔｏｙｏｃａｔ ＥＴ、東ソー製
触媒（３）：Ｔｏｙｏｃａｔ ＥＴＦ、東ソー製
【００２５】
実施例１〜９
（１）ポリイソシアネート組成物の調製
製造例１又は２で得られた有機ポリイソシアネート（イ）又は（ロ）と有機ポリシロキサンとを混合して、ポリイソシアネート組成物を調製した。
（２）ポリオール組成物の調製
製造例３又は４で得られたポリオール予備組成物（イ）又は（ロ）と整泡剤として有機ポリシロキサンとを混合して、ポリオール組成物を調製した。
（３）軟質ポリウレタンフォーム及びフォーム成形品の製造
ポリイソシアネート組成物とポリオール組成物各々を２５℃に調節し、イソシアネートインデックス８５〜１０５になるように計量して、ラボミキサーにより５秒間混合した。これを予め６０℃に加熱したモールドへ注入し発泡させ、５〜６分後に脱型して、軟質ポリウレタンモールドフォーム成形品を得た。
フリー発泡の場合は、ラボミキサーにより５秒間混合し、これをダンボールにポリエチレンシートをセットした中へ注入して、軟質ポリウレタンフリーフォームを得た。
得られた軟質ポリウレタンフォーム及びフォーム成形品の性能について、フリーフォームのセル状態及びモールドフォーム成形品の外観を評価した。
これらの原料組成、配合、フォーム及びフォーム成形品の発泡挙動及び性能をまとめて表３及び４に示す。
【００２６】
比較例１〜６
前記実施例において有機ポリイソシアネート中に有機ポリシロキサンを配合しないことを除いてほぼ同様にして、軟質ポリウレタンフォーム及びフォーム成形品を製造した。
これらの原料組成、配合、フォーム及びフォーム成形品の発泡挙動及び性能をまとめて表５に示す。
【００２７】
【表３】

【００２８】
【表４】

【００２９】
【表５】

表３〜５において、
Ｌ−５３０９：日本ユニカー製、有機ポリシロキサン
ＳＺ−１３０６：日本ユニカー製、有機ポリシロキサン
Ｌ−５３６６：日本ユニカー製、有機ポリシロキサン
沈み率：発泡時最高のフォーム高さになってからのフォーム高さの低下した率
トップハイト：発泡時の最高のフォーム高さ
［軟質ポリウレタンフォーム及びフォーム成形品の性能の測定方法］
フリーフォームの場合はセル状態を、モールドフォーム（成形品）の場合は外観を肉眼で評価した。
判定基準；
◎：非常に良好、○：良好、△：不良、×：著しく不良[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a flexible polyurethane foam having a good appearance and a low defect rate.
[0002]
[Prior art]
In general, when producing a flexible polyurethane foam using an organic polyisocyanate and a polyol, a foam stabilizer is used in combination as one of additives in addition to a foaming agent and a catalyst.
That is, in JP-A-2-14210, a relatively high equivalent polyol, a blowing agent, a high-functional polyether polyol, and a polyol are used in order to produce a (semi) flexible polyurethane foam with little shrinkage and deformation. In addition, a technique for stabilizing these reaction mixtures using a surfactant such as a siloxane / poly (alkylene oxide) copolymer is disclosed. Japanese Patent Laid-Open No. 6-228266 also discloses that a polyurethane foam having a good skin layer and good surface smoothness and elongation can be produced without using chlorofluorocarbon as a foaming agent. In the reaction of a molecular polyol and a specific low-molecular active hydrogen compound with a specific polyisocyanate, a technique is disclosed in which a foam stabilizer such as a polydimethylsiloxane-polyalkylene oxide block polymer is used in combination with a foaming agent and a catalyst. ing. In these techniques, surfactants and foam stabilizers are always blended in polyols because their stability is significantly reduced when blended in organic polyisocyanates.
[0003]
[Problems to be solved by the invention]
However, the appearance of the flexible polyurethane foam molded article produced by the above-mentioned conventionally known method, particularly the flexible polyurethane foam molded article produced by the cold cure mold, is not always satisfactory, and an improvement thereof is desired.
[0004]
The present invention provides a method capable of producing a flexible polyurethane foam having a good appearance of a foam molded article (particularly in cold cure molding) and an extremely low defect rate without using a special polyol. Objective.
[0005]
[Means for Solving the Problems]
As a result of diligent research, the present inventor mixed a specific compound used as a foam stabilizer with a specific organic polyisocyanate and reacted it with a polyol or the like, particularly at the time of cold cure molding. It is possible to improve the mixing property of organic polyisocyanate and polyol, and as a result, it is possible to improve the cell stability of the foam, and to produce a flexible polyurethane foam with a good appearance of the foam molded product and a low defective rate. As a result, the present invention has been completed.
[0006]
That is, the present invention relates to an organic polymer comprising a polyol composition containing a long-chain polyol, a foaming agent, a catalyst, and a foam stabilizer, and diphenylmethane diisocyanate-based polyisocyanate / tolylene diisocyanate = 0.1 to 10.0 (weight ratio). A process for producing a flexible polyurethane foam obtained by reacting an isocyanate and an organic polysiloxane-containing polyisocyanate composition with an isocyanate index of 80 to 110, wherein the diphenylmethane diisocyanate polyisocyanate is 2,2'-diphenylmethane diisocyanate. and / or 2,4'-diphenylmethane diisocyanate and a urethane modified product of diphenylmethane diisocyanate containing a 4,4'-diphenylmethane diisocyanate (isocyanate prepolymer ) And a diphenylmethane diisocyanate polymerization mixture, and the diphenylmethane diisocyanate polymerization mixture comprises 55 to 70% by weight of a polynuclear (polyphenylene polymethylene polyisocyanate) and 45 to 30% by weight of a dinuclear (diphenylmethane diisocyanate). It is a method for producing a flexible polyurethane foam, characterized in that 10-30% by weight of diphenylmethane diisocyanate isomers other than 4,4′-diphenylmethane diisocyanate is contained in the core.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The organic polyisocyanate used in the present invention is a mixture of diphenylmethane diisocyanate (MDI) -based polyisocyanate and tolylene diisocyanate (TDI), and the ratio thereof is MDI-based polyisocyanate / TDI = 0.1 to 10.0 ( (Weight ratio), preferably 0.5 to 9.0 (weight ratio). This MDI-based polyisocyanate is a urethane-modified product of MDI containing 2,2′-MDI and / or 2,4′-MDI and 4,4′- MDI (hereinafter abbreviated as isomer-containing MDI). Isocyanate prepolymer) and diphenylmethane diisocyanate polymerization mixture (Polymec MDI). Polymeric MDI contains 55 to 70% by weight of polynuclear (polyphenylene polymethylene polyisocyanate) and 45 to 30% by weight of dinuclear (MDI), and isomers other than 4,4'-MDI are contained in the two nuclei. It contains 10 to 30% by weight.
In order to achieve the object of the present invention, isomer-containing MDI / TDI = 0.1 to 3.5 (weight ratio) is preferable, and 0.5 to 2.5 (weight ratio) is particularly preferable. Polymeric MDI / TDI = 0.1 to 5.0 (weight ratio) is preferable, and 0.2 to 4.0 (weight ratio) is particularly preferable. Polymeric MDI / isomer-containing MDI = 0.3 to 3.0 (weight ratio) is preferable, and 0.4 to 2.5 (weight ratio) is particularly preferable.
[0008]
In the present invention, the organic polysiloxane blended with the organic polyisocyanate is an organic compound containing a Si—O bond in the molecule, and examples thereof include silicone oil.
Organic polysiloxanes include those containing active hydrogen groups and those containing no active hydrogen groups, but those containing no active hydrogen groups are preferred from the standpoint of solution stability when blended with organic polyisocyanates.
Specific examples include L-5340 manufactured by Union Carbide, SH-193 manufactured by Torre Silicone, TFA-4202 manufactured by Toshiba Silicone, and the like.
The amount of the organic polysiloxane used is 0.05% in the polyisocyanate composition in order to ensure the stability of the blended solution with the organic polyisocyanate and to improve the appearance when a polyurethane foam or foam molded product is formed. ~ 3 wt% is preferred.
[0009]
The long-chain polyol used in the production of the flexible polyurethane foam or foam molded article of the present invention preferably has a molecular weight of 600 to 10,000, particularly 1,000 to 8,000, and preferably 2 to 8 functional groups.
Examples of such long chain polyols include polyether polyols, polyester polyols, polycarbonate polyols, polyether ester polyols, and lactone polyester polyols. These can be used alone or in combination of two or more.
If the molecular weight of the long-chain polyol is less than 600, the polyurethane foam or foam molded product to be produced is hard and brittle, and it is difficult to obtain a flexible and sufficiently strong foam or foam molded product. In view of such performance, polyether polyol and polyester polyol are particularly preferable.
[0010]
Examples of the polyether polyol include ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, 1,4-butanediol, 1,3-butanediol, 1,2-butanediol, 1,5- Pentanediol, 1,7-heptanediol, glycerin, trimethylolpropane, trimethylolethane, hexane-1,2,6-triol, α-methylglucoside, pentaerythritol, sorbitol, sucrose, glucose, fructose, etc. Sugar-based alcohol, bisphenol A, ethylenediamine, propylenediamine, diethylenetriamine, toluenediamine, metaphenylenediamine, diphenylmethanediamine, xylenediamine, etc. With one or more compounds as initiators, one or more monomers such as ethylene oxide, propylene oxide, butylene oxide, amylene oxide, glycidyl ether, methyl glycidyl ether, tert-butyl glycidyl ether, phenyl glycidyl ether are known What is manufactured by addition polymerization by a method can be mentioned. Of these, pentaerythritol-based polyether polyols are particularly preferred from the viewpoint of flexibility and strength of the resulting foam or foam molded product.
[0011]
Examples of the polyester polyol include condensed polyester polyols obtained from acids and alcohols by a known method. Condensed polyester polyols include, for example, saturated or unsaturated dibasic acids such as adipic acid, sebacic acid, terephthalic acid, isophthalic acid, maleic acid and fumaric acid, acid anhydrides such as maleic anhydride and phthalic anhydride, terephthalic acid, etc. It is a polyester polyol obtained by condensation reaction of dialkyl esters such as dimethyl acid with glycols such as ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, neopentyl glycol, and 1,6-hexylene glycol. . Specific examples include polyethylene adipate polyols, polybutylene adipate polyols, polyhexylene adipate polyols and the like, adipate-based polyols composed of one acid and one glycol, polyethylene butylene adipate polyol, polyethylene diethylene adipate polyol, polyhexylene neo Aromatic polyester polyols such as polyethylene adipate terephthalate polyol, polyethylene adipate isophthalate polyol composed of one kind of acid and one kind of glycol such as pentylene adipate polyol, and many kinds of acid and one kind of glycol And so on.
Examples of the polycarbonate polyol include compounds obtained by reacting hexane glycol, 3-methyl-1,5-pentanediol, 1,4-cyclohexanedimethanol and the like with diethyl carbonate, diphenyl carbonate and the like.
Examples of the polyether ester polyol include compounds obtained by reacting the polyether polyol as a polyol component with an aliphatic or aromatic dicarboxylic acid.
Lactone-based polyester polyols can be obtained by ring-opening polymerization of lactones such as ε-caprolactone and γ-butyrolactone, but can be polyols having various compositions depending on the type of initiator. Examples of initiators include glycols such as ethylene glycol and butylene glycol, polyether polyols such as polyoxypropylene glycol (PPG) and polyoxytetramethylene glycol (PTMG), and condensation polyester polyols such as adipate. Can be mentioned.
[0012]
Furthermore, polymer polyols obtained by graft polymerization of the above-mentioned polyether polyols or polyester polyols with ethylenically unsaturated compounds such as acrylonitrile, styrene and methyl methacrylate can also be used.
[0013]
As the foaming agent to be blended with the long-chain polyol to produce the flexible polyurethane foam or foam molded article of the present invention, a known chemical foaming agent and / or physical foaming agent can be used in a usual amount. . For example, the flexible polyurethane foam or foam molded article of the present invention can be obtained by using a low boiling point inert solvent such as acetone, pentane, and hexane, an alternative chlorofluorocarbon such as HCFC-141b, 123, or water.
As a foaming agent in the production of flexible polyurethane foam or foam molded products, water as a carbon dioxide gas generator, and chlorofluorocarbons and low-boiling solvents are commonly used as auxiliary foaming agents that reinforce the foaming action of water. However, in the present invention, it is preferable to use water alone as a foaming agent because criticism of the use of chlorofluorocarbons has increased worldwide due to the problem of ozone layer destruction. The amount of water is preferably 2 to 10 parts by weight, more preferably 3 to 8 parts by weight, based on 100 parts by weight of the long-chain polyol.
[0014]
As the catalyst blended in the long-chain polyol, any known urethanization catalyst can be used and is not particularly limited. For example, as the amine-based urethanization catalyst, triethylamine, tripropylamine, triisopropanolamine, Tributylamine, hexadecyldimethylamine, N-methylmorpholine, N-ethylmorpholine, N-octadecylmorpholine, monoethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N, N-dimethylethanolamine, diethylenetriamine, N, N, N ′, N′-tetramethylethylenediamine, N, N, N ′, N′-tetramethylpropylenediamine, N, N, N ′, N′-tetramethylbutylenediamine, N, N, N ′, N ′ -Tetramethylhe Samethylenediamine, bis [2- (N, N-dimethylamino) ethyl] ether, N, N-dimethylbenzylamine, N, N-dimethylcyclohexylamine, N, N, N ′, N ′, N ″ -penta Methyl diethylenetriamine, triethylenediamine, salts of triethylenediamine, oxyalkylene adducts of amino groups of primary and secondary amines, azacyclo compounds such as N, N-dialkylpiperazines, various N, N ', N "-tri There are alkylaminoalkyl hexahydrotriazines, β-aminocarbonyl catalysts described in JP-B-52-43517, β-aminonitrile catalysts described in JP-B-53-14279, and the like. Tin acetate, octanoate, oleate, tin laurate, dibutyltin dichloride , Lead naphthenate, nickel naphthenate, there are cobalt naphthenate and the like. These catalysts are used alone or in combination. The amount of the catalyst is preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the long-chain polyol.
[0015]
Foam stabilizers incorporated into long chain polyols include, for example, polyglycol ethers containing the required number of alkylene oxides such as ethylene oxide, propylene oxide, butylene oxide, preferably ethylene oxide, and at least one reactivity. What can be obtained by condensing with the organic compound containing a hydrogen atom can be illustrated. Such organic compounds containing at least one reactive hydrogen atom include alcohols, phenols, thiols, primary or secondary amines, carboxylic acids or sulfonic acids, nonionic surfactants that are amides thereof, Moreover, the polyalkylene oxide derivative of the phenol type compound which has 1 or more alkyl group substitution group etc. can be mentioned.
Furthermore, preferred foam stabilizers in the present invention can include pluronic surfactants, such as butylene oxide, amylene oxide, phenylethylene oxide, cyclohexene oxide, propylene oxide, or mixtures thereof, It is obtained by polymerizing 1,2-alkylene oxide or substituted alkylene oxide in the presence of an alkali catalyst to produce a corresponding water-insoluble polyalkylene glycol and condensing it with the required number of moles of ethylene oxide under the same conditions. It is a nonionic surfactant.
Furthermore, it is necessary for alcohols obtained by reducing aldehydes generated by catalytic reaction of polyolefins such as tripropylene, tetrapropylene, pentapropylene, ditributylene, triisobutylene, propyleneisobutylene and tributene with carbon monoxide and hydrogen. Nonionic surfactants obtained by reacting a small number of moles of ethylene oxide can be exemplified.
Moreover, the organic polysiloxane blended with the organic polyisocyanate can also be cited as an example of a preferable compound for the foam stabilizer blended with the long-chain polyol.
The blending amount of the foam stabilizer in the long chain polyol is preferably 3 parts by weight or less with respect to 100 parts by weight of the long chain polyol.
[0016]
In the method for producing a flexible polyurethane foam or foam molded article of the present invention, the polyol composition and the polyisocyanate composition are reacted at an isocyanate index of 80 to 110, and particularly preferably at 85 to 105. This reaction can be performed, for example, by mixing both compositions at 20-30 ° C., stirring for 2-6 seconds, and then in a mold at 40-70 ° C.
[0017]
In the present invention, known emulsifiers, antioxidants such as antioxidants and ultraviolet absorbers, fillers such as calcium carbonate and barium sulfate, flame retardants, plasticizers, colorants, antifungal agents and the like are known. Various additives, auxiliaries and the like can be used in combination as necessary.
[0018]
【The invention's effect】
As described above, a flexible polyurethane foam or foam molded article produced by the method of the present invention using a polyisocyanate composition having excellent solution stability despite containing an organic polysiloxane is molded. Good appearance and extremely low defect rate. In particular, when the present invention is applied to cold cure molding, the performance of the resulting flexible polyurethane foam and foam molded product is remarkably excellent.
[0019]
【Example】
EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to this.
In Examples and Comparative Examples, “parts” and “%” respectively represent “parts by weight” and “% by weight” unless otherwise specified.
[Preparation of organic polyisocyanate]
Production Example 1
Polyisocyanate (1) and polyols (1), (2) and (4) were reacted at 70 ° C. for 3 hours to obtain an isocyanate prepolymer. Next, polyisocyanate (2) and (3) were added and mixed with this, and organic polyisocyanate (I) was obtained.
Table 1 summarizes the raw material composition, the isocyanate (NCO) content and viscosity of the organic polyisocyanate (I).
[0020]
Production Example 2
Organic polyisocyanate (b) was obtained in the same manner as in Production Example 1 except that polyols (3) and (4) were used instead of polyols (1), (2) and (4).
Table 1 summarizes the raw material composition and the NCO content and viscosity of the organic polyisocyanate (B).
[0021]
[Table 1]

In Table 1,
Polyisocyanate (1): MDI containing 18% isomer
Polyisocyanate (2): 60% polynuclear, 40% MDI, 15% isomer in MDI
Polyisocyanate (3): TDI (Coronate T-80, manufactured by Nippon Polyurethane Industry)
Polyol (1): molecular weight 8000, functional group number 4, polyether polyol Polyol (2): molecular weight 2000, functional group number 2, polyether polyol Polyol (3): molecular weight 1000, functional group number 2, polyether polyol Polyol (4): Molecular weight 2550, functional group number 2, polyester polyol
[Preparation of polyol pre-composition]
Production Example 3
Polyols (6), (9) and (10), catalysts (1) and (3), and water were added and mixed in this order to obtain a polyol preliminary composition (I).
The raw material composition is summarized in Table 2.
[0023]
Production Example 4
Polyols (5), (6), (7) and (8), catalysts (1) and (2), and water were added in this order and mixed to obtain a polyol preliminary composition (B).
The raw material composition is summarized in Table 2.
[0024]
[Table 2]

In Table 2,
Polyol (5): Molecular weight 7000, functional group number 3, polyether polyol Polyol (6): Molecular weight 7000, functional group number 4, polyether polyol Polyol (7): Polymer polyol composed of acrylonitrile and styrene Polyol (8): Molecular weight 1500 , Functional group number 3, polyether polyol polyol (9): molecular weight 6000, functional group number 3, polyether polyol polyol (10): polymer polyol made of acrylonitrile Catalyst (1): TEDA L-33, manufactured by Tosoh Catalyst (2) : Toyocat ET, manufactured by Tosoh Catalyst (3): Toyocat ETF, manufactured by Tosoh [0025]
Examples 1-9
(1) Preparation of polyisocyanate composition The organic polyisocyanate (A) or (B) obtained in Production Example 1 or 2 and an organic polysiloxane were mixed to prepare a polyisocyanate composition.
(2) Preparation of Polyol Composition A polyol composition was prepared by mixing the polyol pre-composition (A) or (B) obtained in Production Example 3 or 4 and organic polysiloxane as a foam stabilizer.
(3) Production of Flexible Polyurethane Foam and Foam Molded Article Each of the polyisocyanate composition and the polyol composition was adjusted to 25 ° C., weighed to have an isocyanate index of 85 to 105, and mixed for 5 seconds by a laboratory mixer. This was poured into a mold preheated to 60 ° C. to cause foaming, and demolded after 5 to 6 minutes to obtain a molded product of a flexible polyurethane mold.
In the case of free foaming, it was mixed for 5 seconds with a lab mixer, and this was poured into a cardboard set with a polyethylene sheet to obtain a flexible polyurethane free foam.
About the performance of the obtained flexible polyurethane foam and foam molded article, the cell state of the free foam and the appearance of the molded foam molded article were evaluated.
Tables 3 and 4 collectively show the raw material composition, blending, foaming behavior and performance of the foam and the molded article.
[0026]
Comparative Examples 1-6
A flexible polyurethane foam and a foam molded article were produced in substantially the same manner except that no organic polysiloxane was blended in the organic polyisocyanate in the above examples.
Table 5 summarizes the raw material composition, blending, foam and foaming behavior and performance of the foam molded product.
[0027]
[Table 3]

[0028]
[Table 4]

[0029]
[Table 5]

In Tables 3-5,
L-5309: Made by Nihon Unicar, Organic Polysiloxane SZ-1306: Made by Nihon Unicar, Organic Polysiloxane L-5366: Made by Nihon Unicar, Organic Polysiloxane Sinking rate: Foam height after reaching the highest foam height during foaming Top height: Maximum foam height when foaming [Method for measuring the performance of flexible polyurethane foam and foam moldings]
The cell state was evaluated with the naked eye in the case of free form, and the appearance was evaluated with the naked eye in the case of mold foam (molded product).
Judgment criteria;
◎: Very good, ○: Good, △: Bad, ×: Extremely bad

Claims (1)

Organic polyisocyanate and organic polysiloxane comprising a polyol composition containing a long-chain polyol, a foaming agent, a catalyst and a foam stabilizer, and diphenylmethane diisocyanate-based polyisocyanate / tolylene diisocyanate = 0.1 to 10.0 (weight ratio) A process for producing a flexible polyurethane foam obtained by reacting a polyisocyanate composition containing a polyisocyanate composition with an isocyanate index of 80 to 110,
The diphenylmethane diisocyanate-based polyisocyanate contains 2,2′-diphenylmethane diisocyanate and / or 2,4′-diphenylmethane diisocyanate and 4,4′-diphenylmethane diisocyanate, a urethane-modified diphenylmethane diisocyanate (isocyanate prepolymer), Diphenylmethane diisocyanate polymerization mixture, and the diphenylmethane diisocyanate polymerization mixture is composed of 55 to 70% by weight of polynuclear (polyphenylene polymethylene polyisocyanate) and 45 to 30% by weight of dinuclear (diphenylmethane diisocyanate). Containing 10 to 30% by weight of an isomer of diphenylmethane diisocyanate other than 4,4'-diphenylmethane diisocyanate, The process for producing a flexible polyurethane foam which is characterized.