JP3281662B2 - Partial prepolymer and flexible polyurethane foam using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a partial prepolymer having a molecular terminal NCO group used for producing a flexible polyurethane foam having improved rebound resilience, which is useful as a seat cushion for automobiles and furniture. And a flexible polyurethane foam using the same.

[0002]

2. Description of the Related Art A flexible polyurethane foam is widely used as a seat / cushion material for furniture, automobiles, trains, etc., especially due to its high elasticity. Conventional seat
As the cushioning material, block foams that are industrially mass-produced as soft polyurethane slab foams are cut into necessary sizes and used, or a molding method using a so-called hot cure method or a cold cure method is used. It has been produced in. In all of these, toluene diisocyanate (hereinafter abbreviated as TDI) has been mainly used as a polyisocyanate, but the production method of a flexible polyurethane foam by a cold cure method is capable of high-speed curing compared to a hot cure method. And have high productivity. In recent years, diphenylmethane diisocyanate (hereinafter referred to as MDI) has been developed as a system capable of higher speed curing and higher productivity.
The so-called MDI system using a partial prepolymer of a molecular terminal NCO group whose main component is abbreviated as "(1)" has attracted attention from the viewpoint of improving the working environment due to its high productivity and low vapor pressure. T as polyisocyanate
Flexible foams using MDI instead of DI have been proposed.

For example, 4,4'-MDI of Japanese Patent Publication No. 58-458,
A method using a mixture of 2,4'-MDI and crude MDI, a method using a urethane-modified MDI with a diol disclosed in JP-B-60-15649, and a urethane-modified MDI using three glycols disclosed in JP-B-58-13567. Method, Japanese Patent Publication No. 60-2197
7, a method in which MDI is modified using a diol having a molecular weight of 175 or less, a method in which MDI is modified with a monool described in JP-B-56-92855, and a method wherein the number of functional groups is 2 as described in JP-B-63-38369.
4, a method of modifying MDI with a polyol having a molecular weight of 1500 or less, and 4,4′-MDI and 2,4'-MDI with a polyoxyalkylene polyol having 2 to 3 functional groups according to USP 4,261,852.
Modification of 4'-MDI mixture and mixing with crude MDI, US Pat. No. 4,478,960, method of modifying crude MDI with 5-30% polyoxyalkylene polyol in polyoxyalkylene polyol Is mentioned. However, in the conventional method, when a flexible polyurethane foam is used as a seat cushion, the rebound resilience, which is a measure of the cushion feeling, which is one of the basic performances, is smaller than that of a cold-cured flexible polyurethane foam using TDI. However, there is a drawback that the temperature is significantly reduced, and an improvement thereof is required.

[0004]

DISCLOSURE OF THE INVENTION The present invention relates to a cushioning feeling, a repulsion, which is the basic performance of a cold-cure flexible polyurethane foam using MDI as a seat cushion material while maintaining high-speed curing property and productivity. The purpose is to improve the elasticity.

[0005]

Means for Solving the Problems To solve these problems, the present inventors have increased the molecular weight and the number of functional groups of the polyoxyalkylene polyol which modifies MDI and reduced the content of polyoxyethylene groups. Have found that it is possible to improve the rebound resilience of a flexible polyurethane foam obtained from a partial prepolymer having a molecular terminal NCO group, and completed the present invention. That is, the present invention

(1) 4,4′-diphenylmethane diisocyanate, 2,4′-
4,4′-diphenylmethane diisocyanate in which 20 to 90% by weight of 4,4′-diphenylmethane diisocyanate containing one or two or more selected from the group consisting of dinuclear methane diisocyanate and trinuclear or more components in the crude diphenylmethane diisocyanate, 2,4'-
Polyisocyanate mixture 1 containing 5 to 35% by weight of diphenylmethane diisocyanate and 2 to 30% by weight of a trinuclear or higher component in the crude diphenylmethane diisocyanate 1
00 parts by weight, hydroxyl value of 15 to 90 mgKOH / g, number of functional groups of initiator of 3 to 6, polyoxyethylene in polyoxyalkylene polyol obtained from the initiator (hereinafter abbreviated as EO)
The content of the NCO group at the molecular terminal for a flexible polyurethane foam obtained by reacting 10 to 50 parts by weight of at least one polyoxyalkylene polyol having a group content of 5% by weight or less and the balance being a polyoxypropylene group. polymer.

(2) A flexible polyurethane obtained by reacting the partial prepolymer having a molecular terminal NCO group according to item 1 with a polyoxyalkylene polyol in the presence of water, a catalyst, a silicone surfactant, a crosslinking agent and other auxiliaries. Provides a form. The auxiliaries refer to foaming auxiliaries, cocatalysts, coloring agents, fillers, stabilizers and the like. As the 4,4'-diphenylmethane diisocyanate (4,4'-MDI), for example, Cosmonate MDI-PH manufactured by Mitsui Toatsu Chemicals, Inc. can be exemplified. Crude MDI refers to other mixtures of 40 to 60% by weight of the trinuclear or higher nucleus of Formula 1 and 60 to 40% by weight of 4,4'-MDI. This is usually crude MDI, polymeric M
Supplied under the name DI or polymethylene polyphenyl polyisocyanate. Crude MDIs commonly used in the art also include 1-5% by weight of 2,4'-MDI,
Less than 2% 2,2'-MDI, carbodiimide modified M
DI. As the crude MDI, for example, Cosmonate M-200 manufactured by Mitsui Toatsu Chemical Co., Ltd. can be exemplified (for example, 4,4′-MDI 42.3% by weight, 2,4′-M
DI 2.4% by weight, 2,2'-MDI 0.3% by weight, trinuclear or higher components 55% by weight).

The crude MDI of the present invention, 4,4'-MDI, 2,4'-
A polyoxyalkylene polyol to be reacted with MDI or a mixture of these isomers and components of three or more nuclei (polyisocyanate mixture) is used as a low-molecular polyhydroxy compound such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. It is a conventionally known product obtained by addition polymerization of propylene oxide or propylene oxide and ethylene oxide in the presence of a basic catalyst.

Embedded image (Here, n ≧ 1) The content of 4,4′-MDI in the polyisocyanate mixture is 20 to 90% by weight, preferably 25 to 85% by weight, and more preferably 30 to 80% by weight. . When the content of 4,4′-MDI is less than 20% by weight, a flexible polyurethane foam using the obtained partial prepolymer having a molecular terminal NCO group (hereinafter abbreviated as a partial prepolymer) is caused to shrink,
If the 4,4'-MDI content exceeds 90% by weight, the low-temperature stability of the partial prepolymer obtained therefrom deteriorates,
Solidifies at room temperature.

2,4'- in the above polyisocyanate mixture
The content of MDI is 5-35% by weight. If the content of 2,4'-MDI is less than 5% by weight, the stability of the flexible polyurethane foam obtained therefrom will be reduced, leading to foam collapse and the content of 2,4'-MDI exceeding 35% by weight. in case of,
The curing rate (curing property) of the flexible polyurethane foam obtained therefrom is significantly retarded (deteriorated), and MDI
High productivity, which is a feature of flexible polyurethane foam, is impaired. The content of the compound having three or more nuclei in the polyisocyanate mixture is 2 to 30% by weight, preferably 8 to 30% by weight.
25% by weight. When the content of the trinuclear or higher component is less than 2% by weight, the stability of the flexible polyurethane foam obtained therefrom is reduced, and the foam collapses. If the content of the three or more nuclei exceeds 30% by weight, the elastic polyurethane foam obtained therefrom will have remarkably reduced elongation and tear strength, and the durability required for a vehicle cushion cannot be satisfied. 4,4'-MDI, 2,4, -MDI
4′-MDI, a mixture of three or more nuclei or more is appropriately blended to prepare a polyisocyanate mixture having a desired weight ratio.

The polyoxyalkylene polyol to be reacted with the above polyisocyanate mixture has a hydroxyl value of 15 to 90 mg.
KOH / g, preferably 20-85 mg KOH / g, more preferably
The polyoxyalkylene polyol has a functional group number of 3 to 6, preferably 3 to 5, and more preferably 3 to 4. Even if the partial prepolymer of the present invention is produced using a polyoxyalkylene polyol having a hydroxyl value of less than 15 mgKOH / g, improvement in the resilience of a flexible polyurethane foam obtained from the partial prepolymer cannot be expected. When the partial prepolymer of the present invention is produced using a polyoxyalkylene polyol having a hydroxyl value exceeding 90 mgKOH / g, the resilience of a flexible polyurethane foam obtained from the partial prepolymer is remarkably reduced. When the partial prepolymer of the present invention is produced using a polyoxyalkylene polyol having less than 3 functional groups, the foaming stability of the flexible polyurethane foam obtained from the partial prepolymer decreases,
Before foaming and hardening, the flexible polyurethane foam collapses, and a desired flexible polyurethane foam cannot be obtained. Further, when the partial prepolymer of the present invention is produced using a polyoxyalkylene polyol having a number of functional groups exceeding 6, the elongation and tear strength of the flexible polyurethane foam obtained from the partial prepolymer are remarkably reduced. Four,
The content of EO groups in the polyoxyalkylene polyol to be reacted with 4′-MDI, 2,4′-MDI and other isomers and a mixture of three or more nuclei is 5% by weight based on the total weight of the polyoxyalkylene polyol. When the content of the polyoxyethylene group exceeds 5% by weight, when the partial prepolymer of the present invention is produced using the polyoxyalkylene polyol, the soft polyurethane obtained from the partial prepolymer is used. The resilience of the foam is significantly reduced.

When reacting 100 parts by weight of a mixture of 4,4'-MDI, 2,4'-MDI and a trinuclear or higher component with a polyoxyalkylene polyol, the polyoxyalkylene polyol is preferably 10 to 50 parts by weight. , More preferably 15
~ 45 parts by weight. NCO of the obtained partial prepolymer
% Is in a range suitable for the present invention from 19 to 30%. If the amount of the polyoxyalkylene polyol to be reacted is less than 10 parts by weight, the low-temperature stability of the above partial prepolymer deteriorates, the coagulation temperature rises, and handling in winter becomes a problem, and foaming of the flexible polyurethane foam obtained therefrom becomes problematic. Since the time stability also decreases, a phenomenon in which the foam collapses is observed. The amount of the polyoxyalkylene polyol to be reacted is 50
If the amount is more than 10 parts by weight, the density of the flexible polyurethane foam using the partial prepolymer obtained therefrom is increased, and the closed cell rate of the foam is increased, so that the air permeability of the foam is significantly reduced.

A reaction product obtained by reacting diphenylmethane diisocyanate with a polyoxyalkylene polyol is added to a crude MDI containing various isomers of MDI.
When a polyisocyanate mixed with is manufactured, the stability at the time of foaming of the flexible polyurethane foam obtained therefrom is reduced, and the foam collapses during foaming.
Substantially no foam can be produced. Therefore, in the present invention, 4,4′-MDI, 2,4′-MDI and other isomers and components of three or more nuclei are reacted with the polyoxyalkylene polyol described in claim 1 of the claims. Is an essential condition. The effect of the present invention is not impaired even if carbodiimide or uretonimine-modified MDI or crude MDI is further mixed and used as needed with the partial prepolymer having a molecular terminal NCO group of the present invention.

Water during the production of a flexible polyurethane foam reacts with an organic polyisocyanate to form a urea-binding component, and generates carbon dioxide and acts as a foaming agent. Is indispensable. The amount of water used is not particularly limited, but is preferably 1 to 8 parts by weight, more preferably 1.5 to 6 parts by weight, based on 100 parts by weight of the polyol component used. Foaming aids other than water during the production of flexible polyurethane foam include CFC-11, HCFC-123, HCF
In addition to C-141b, for example, dichlorodifluoromethane (CFC-12), difluorochloromethane (HCFC-
22), at least one of halocarbons of methane and ethane, such as tetrafluoroethane (HFC-134a or its isomer HFC-134), and other known non-halogens Compounds can also be used as foaming aids. The amount of the foaming aid used is not particularly limited, but the polyol component 100
It is used in an amount of 1 to 20 parts by weight based on parts by weight.

The catalyst used in the present invention is known in the art and is not particularly limited. Examples of the amine catalyst include bis [2- (N, N-dimethylamino) ethyl] ether and triethylenediamine. can give. The silicone surfactant used in the present invention is a conventionally known organosilicon surfactant, for example, L-L manufactured by Nippon Unicar Co., Ltd.
5309 mag., SRX manufactured by Torre Dow Corning
-274 C, SF-2962 and the like, and TEGOSTAB B-4113 and the like manufactured by Goldschmidt. These surfactants are used alone or as a mixture, and the amount is 0.1 to 5.0 parts by weight based on 100 parts by weight of the polyol. Examples of the crosslinking agent used in the present invention include alkanolamines such as triethanolamine and diethanolamine, low molecular weight polyols such as glycerin, and hydroxyl values obtained by adding ethylene oxide, propylene oxide and the like to these active hydrogen compounds. Compounds of 200 mgKOH / g or more can be mentioned.

These crosslinking agents are used alone or as a mixture. The amount of the crosslinking agent is 0.1 to 100 parts by weight of the polyol.
15.0 parts by weight. The polyoxyalkylene polyol used in the production of the flexible polyurethane foam is water, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, trimethylene glycol, glycerin, 1,1,1
-Trimethylolpropane, pentaerythritol, sorbitol, sucrose, lactose, polyhydric hydroxy compounds such as novolak resin, ethylenediamine, diethylenetriamine, polyamines such as triethylenetetramine, triethanolamine, alkanolamines such as diethanolamine, ethylene oxide, It is a conventionally known polyether polyol obtained by adding one or more of propylene oxide, butylene oxide, tetrahydrofuran, styrene oxide and the like. The preferred range of the hydroxyl value of the various polyoxyalkylene polyols described above is 15 to 150 mgKOH / g,
These polyols are used alone or as a mixture.
In the present invention, in addition to the above, various stabilizers, fillers, coloring agents, flame retardants and the like can be used as required.

In producing a flexible polyurethane foam, a crosslinker, a foam stabilizer, water, a foaming aid, and a catalyst are mixed with a polyoxyalkylene polyol as necessary, and a resin premix is prepared. Mix with the partial prepolymer and react and cure. The equivalent ratio of NCO groups and OH groups during the production of the flexible polyurethane foam in the present invention, that is, the range of the NCO index is 0.50 to 1.50,
Preferably it is 0.70 to 1.30.

[0017]

The present invention will be specifically described below with reference to examples and comparative examples. In the following, parts and percentages are by weight unless otherwise specified. The weight percent of NCO groups in the isocyanate compound is referred to as NCO%. The measurement of various physical properties of the flexible polyurethane foam was performed in accordance with JIS K-6401 and JIS K-6301. The following materials were used in the examples. Polyol A: a polyoxyalkylene triol having a hydroxyl value of 28 mgKOH / g obtained by subjecting propylene oxide to ring-opening addition polymerization of glycerin and further subjecting ethylene oxide to ring-opening addition polymerization. Polymer polyol A: a polymer polyol having a hydroxyl value of 28 mgKOH / g obtained by radical polymerization of acrylonitrile and styrene in polyol A. Water: ion-exchanged water. DEOA: diethanolamine. SRX-274 C: Silicone surfactant manufactured by Toray Dow Corning. Minico L-1020: Active Material Chemical Co., Ltd. catalyst (33% solution of triethylenediamine in tripropylene glycol) Minico TMDA: Active Material Chemical, catalyst (bis (N, N-dimethylaminoethyl) ether) 4 , 4'-MDI: Cosmonate MDI-PH (NCO% 33.6%) manufactured by Mitsui Toatsu Chemical Co., Ltd. 2,4'-MDI: Cosmonate MDI-PH (NCO%) manufactured by Mitsui Toatsu Chemical Co., Ltd.
33.6%) Crude MDI: Cosmonate M manufactured by Mitsui Toatsu Chemicals, Inc.
-200 (NCO% 31.0%, 4,4'-MDI 42.3%, 2,
4'-MDI 2.4%, 2,2'-MDI 0.3% and 3
Contains 55% or more of the core component)

4,4'- into a 5-liter separable flask
MDI, 2,4'-MDI, trinuclear or higher components and crude MDI
Are mixed in amounts shown in Tables 1, 2 and 3 and completely dissolved and mixed at 50 ° C. Then, the hydroxyl values shown in Tables 1, 2, 3,
A polyoxyalkylene polyol having a functional group number and a polyethylene oxide content was added in the amounts shown in Tables 1, 2 and 3, and the temperature was raised to 80 ° C. with stirring, followed by reaction for 90 minutes to obtain a partial prepolymer. NCO% of partial prepolymer is
24-28%. Separately, 80 parts of polyol A, 20 parts of polymer polyol A, 3.4 parts of water, 1.0 part of DEOA,
SRX-274C 1.0 part, Minico L-1020
0.6 parts and 0.2 parts of Minico TMDA were stirred and mixed to obtain a resin premix A, and the temperature was adjusted to 25 ° C.
The previously synthesized partial prepolymer and resin premix A, which had been temperature-controlled to 25 ° C., were vigorously stirred and mixed for 6 seconds at the ratios shown in Tables 1, 2, and 3 to produce a flexible polyurethane foam. Free foaming was performed at atmospheric pressure in a 3 liter polypropylene vat. In the free foaming, the foaming stability and the presence or absence of collapse of the foam were visually evaluated with reference to the foaming. Mold foam is pre-heated to 60 ° C, coated with a commercial release agent, size
Using a 350 × 350 × 100 mm aluminum test mold, the layer of the reaction mixture was adjusted so that the target density was attained, the reaction mixture was injected, the lid was closed, the container was closed with a clamp, and the foam was cured. After 3 minutes from the injection of the mold, the clamp of the mold is released, the cured flexible polyurethane foam is released from the mold, and the crushing process is used to form open cells, and the presence or absence of deformation due to insufficient curing is visually evaluated.
Various physical properties were measured in accordance with SK-6301 and JIS K-6401.

(Examples 1 to 6, Comparative Examples 1 and 2) Example 1
~ 6 (Table 1), hydroxyl value 15 ~ 90mgKOH / g
The rebound resilience of a flexible polyurethane foam made of a partial prepolymer produced using a mixture of 4,4'-MDI, 2,4'-MDI and crude MDI, which is urethane-modified with a polyoxyalkylene polyol, is usually required for a vehicle cushion. Shows a value higher than 55%, but a hydroxyl value of 15 mgKOH / g
Flexible polyurethane foams made with partial prepolymers urethane modified with less than polyoxyalkylene polyol collapsed. (Table 1 Comparative Example 1). Also,
As shown in Comparative Example 2 (Table 1), the hydroxyl value was 90 mgKOH / g.
A flexible polyurethane foam made of a partial prepolymer using a polyoxyalkylene polyol having a hydroxyl value of more than 40% has a rebound resilience reduced to 41%, and is not suitable as a vehicle cushion material.

(Examples 1 to 6, Comparative Examples 3 and 4) Example 1
As shown in Tables 1 to 6, the rebound resilience of a flexible polyurethane foam produced by using a partial prepolymer modified with a urethane-modified polyoxyalkylene polyol having a polyoxyethylene group content of 5% by weight or less is usually less than that of a vehicle. Of a flexible polyurethane foam produced using a partial prepolymer which is urethane-modified with a polyoxyalkylene polyol having a polyoxyethylene group content of more than 5% by weight, which value is higher than 55% required for a cushion for use. The rebound resilience is reduced to 50% (Comparative Examples 3 and 4, Table 1) and is not suitable as a vehicle cushion material.

(Examples 7 to 9, Comparative Examples 5 and 6) Example 7
~ 9 (Table 2), as shown in 4,4'-MDI, 2,4'-M
The number of functional groups of the initiator of the polyoxyalkylene polyol using urethane modification of a mixture of DI and crude MDI is 3 to
The tear strength of the flexible polyurethane foam produced using the partial prepolymer which is No. 6 is 0.50 to 0.60 kg / cm 2,
A flexible polyurethane foam made of a partial prepolymer using a polyoxyalkylene polyol synthesized using an initiator having two functional groups satisfies the tear strength of 0.5 kg / cm normally required for a cushion material. As shown in 2), the stability during foaming was reduced, and the foam collapsed during foaming, making it difficult to produce a foam. Further, as shown in Comparative Example 6 (Table 2), a flexible polyurethane foam made of a partial prepolymer using a polyoxyalkylene polyol synthesized using an initiator having 8 functional groups has a tear strength of up to 0.42 kg / cm. This causes problems in durability and toughness as a cushion material.

(Examples 10 and 11, Comparative Examples 7 and 8) Example 1
As shown in Tables 0 and 11 (Table 3), 4,4′-MDI, 2,4′-
There is no problem with respect to elongation, tear strength and rebound resilience of a flexible polyurethane foam produced using a partial prepolymer having a content of tri- or higher nuclei or more in a mixture of MDI and crude MDI of 2 to 30% by weight. As shown in Comparative Example 7 (Table 3), when the content of components of three or more nuclei exceeds 30% by weight, the elongation and tear strength of the flexible polyurethane foam are greatly reduced, and the mechanical strength required for the cushion material is reduced. I'm not satisfied. Further, the production of a flexible polyurethane foam using a partial prepolymer containing no trinuclear or higher component was tried. However, the stability of the foam during foaming was reduced, and the foam collapsed during foaming. Was difficult to manufacture. (Comparative Example 8, Table 3).

(Examples 12 and 13, Comparative Examples 9 and 10)
~ 13 (Table 3), as shown in 4,4'-MDI, 2,4'-M
For partial prepolymers with a content of 2,4'-MDI of 5 to 35% by weight in the mixture of DI and crude MDI, there is no problem in the performance of the flexible polyurethane foams produced therefrom, in 3 minutes from the injection of the mold. Even if the flexible polyurethane foam is removed from the mold (demolding), the degree of curing is sufficient, and the crushing process (cells that remain as closed cells are crushed by a roller to 80% of the original thickness to form physically open cells. No deformation of the outer shape is observed at all. However, when a flexible polyurethane foam is produced using a partial prepolymer having a 2,4′-MDI content of more than 35% by weight, the curing speed of the foam is reduced (so-called cure property is reduced), and three minutes after injection of the mold. The degree of curing when the flexible polyurethane foam is removed from the mold (demolding) is insufficient, the area crushed by hand is deformed, and the deformation is not recovered (so-called finger marks remain), so curing for a longer time It turns out that time is required (Comparative Example 9, Table 3). This is MD
This is a property that impairs the high productivity of the I-based flexible foam, and such a defect is unbearable. 2,4′-MD in Comparative Example 10 (Table 3)
The results of the production of a flexible polyurethane foam with a partial prepolymer synthesized without adding any I are shown. 2,4 '
-When a partial prepolymer containing no MDI was used, the stability of the flexible polyurethane foam during foaming was reduced, and the foam collapsed during foaming, making it difficult to produce a flexible polyurethane foam.

(Comparative Example 11) It is clear that the flexible polyurethane foam produced by using the partial prepolymer of the present invention has excellent rebound resilience and mechanical strength and does not lower the productivity, but 4,4'-MDI. 56.2 parts by weight and 2,4'-MD
I 17.7 parts by weight of the mixture was modified with 30.1 parts by weight of the polyalkylene polyol shown in Example 1 (Table 1) (under the same conditions as in Example 1), and then 26.0 parts by weight of crude MDI was added and mixed after completion of the reaction. Comparative Example 11 will be described. Production of a flexible polyurethane foam using the thus-prepared partial prepolymer was attempted using Resin Premix A in the same liquid ratio and method as in Example 1 (Table 1).
The stability of the foam during foaming was reduced, and the foam collapsed during foaming, making it difficult to produce a flexible polyurethane foam.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

EFFECTS OF THE INVENTION While maintaining high productivity in the production of conventional MDI-based flexible polyurethane foam, rebound resilience,
A flexible polyurethane foam having improved mechanical strength can be obtained.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Seijiro Sakai 1190 Kasama-cho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Inside Mitsui Toatsu Chemicals Co., Ltd. (56) References JP-A-3-17115 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) C08G 18/00-18/87

Claims (2)

(57) [Claims] 1. A crude diphenylmethane diisocyanate is added to 4,4′-diphenylmethane diisocyanate, 2,4′-
4,4′-diphenylmethane diisocyanate in which 20 to 90% by weight of 4,4′-diphenylmethane diisocyanate containing one or two or more selected from the group consisting of dinuclear methane diisocyanate and trinuclear or more components in the crude diphenylmethane diisocyanate, 2,4'-
Polyisocyanate mixture 1 containing 5 to 35% by weight of diphenylmethane diisocyanate and 2 to 30% by weight of a trinuclear or higher component in the crude diphenylmethane diisocyanate 1
00 parts by weight, a hydroxyl value of 15 to 90 mgKOH / g, the number of functional groups of the initiator is 3 to 6, the content of the polyoxyethylene group in the polyoxyalkylene polyol obtained from the initiator is 5% by weight or less, and the balance is A partial prepolymer of a molecular NCO group for a flexible polyurethane foam obtained by reacting 10 to 50 parts by weight of at least one polyoxyalkylene polyol comprising oxypropylene groups. 2. The method according to claim 1, wherein the partial prepolymer having a molecular terminal NCO group and a polyoxyalkylene polyol are water,
A flexible polyurethane foam obtained by reacting in the presence of a catalyst, a silicone surfactant, a crosslinking agent and other auxiliaries.