JP2007056212A - Flexible polyurethane foam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a flexible polyurethane foam prevented from cracking while maintaining its curability and shrinkage. <P>SOLUTION: The flexible polyurethane foam 13 is obtained by reaction of feedstocks comprising a polyol, a polyisocyanate, a catalyst, water as foaming agent and a foam stabilizer at 120-150°C to effect expansion and curing. This polyurethane foam 13 has an apparent density of 20-30 kg/m<SP>3</SP>and an expansion height H ratio defned below of 0-3%. Expansion height ratio H=ä[(expansion height H<SB>1</SB>at 50°C)-(expansion height H<SB>2</SB>at 20°C)]/(expansion height H<SB>2</SB>at 20°C)}×100. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a flexible polyurethane foam used, for example, as a material for automobile seats, furniture, bedding and the like.

  Conventionally, this type of flexible polyurethane foam includes a so-called hot foam (hot foam) obtained by foaming at a high temperature of 120 to 150 ° C. and a cold foam (cold foam) obtained by foaming at a low temperature of 60 to 80 ° C. Foam). In hot foam, the cell is more likely to be closed (sealed) than in cold foam, so that foaming gas tends to remain in the cell. Therefore, a phenomenon in which the foam bursts (hereinafter referred to as a crack) is observed at a portion where such cells are concentrated.

In order to prevent such cracks, for example, a technique using an amine compound having a specific structure without using a tin catalyst as a catalyst has been proposed (see, for example, Patent Document 1). Such amine compounds are six kinds of compounds containing at least one primary amino group, secondary amino group or hydroxyalkyl group in the molecule.
JP 2004-27010 A (pages 2 to 8)

  By the way, at the time of producing a flexible polyurethane foam, factors such as temperature, polyols, polyisocyanates, water as a foaming agent, foam stabilizer, catalyst and the like can be considered as factors for controlling the foaming state. Only when these conditions are optimally combined, the progress of foaming and the formation of cells can be well controlled. Accordingly, as disclosed in Patent Document 1, the use of a specific amine compound as a catalyst cannot sufficiently control the progress of foaming and the formation of cells. As a result, since the formation of the cells is not performed in a stable state, the cells are closed in the foaming process, and the cracks cannot be sufficiently suppressed. There was a problem that shrinkability may be reduced.

  Accordingly, an object of the present invention is to provide a flexible polyurethane foam capable of suppressing cracks while maintaining curability and shrinkability.

In order to achieve the above object, the flexible polyurethane foam of the invention described in claim 1 is obtained by using a raw material of a flexible polyurethane foam containing polyols, polyisocyanates, a catalyst, water as a foaming agent, and a foam stabilizer. It is a flexible polyurethane foam obtained by reacting, foaming and curing at 120 to 150 ° C. and having an apparent density of 20 to 30 kg / m ³ , and the foam height ratio shown below is 0 to 3%. It is characterized by.

Ratio of foaming height = {[(foaming height at 50 ° C.) − (Foaming height at 20 ° C.)] / (Foaming height at 20 ° C.)} × 100
The flexible polyurethane foam of the invention according to claim 2 is characterized in that, in the invention according to claim 1, the amount of water is 5 to 8 parts by mass per 100 parts by mass of polyols.

  The flexible polyurethane foam of the invention according to claim 3 is the invention according to claim 1 or claim 2, wherein the blending amount of the foam stabilizer is 1.7 to 1.9 parts by mass per 100 parts by mass of polyols. It is characterized by being.

  A flexible polyurethane foam according to a fourth aspect of the present invention is the flexible polyurethane foam according to any one of the first to third aspects, wherein the catalyst is composed of a resination catalyst and a foaming catalyst, and the blending amount of the resinization catalyst Is 0.15 to 0.25 parts by mass per 100 parts by mass of polyols, and the blending amount of the foaming catalyst is 0.05 to 0.2 parts by mass per 100 parts by mass of polyols. is there.

According to the present invention, the following effects can be exhibited.
In the flexible polyurethane foam of the invention according to claim 1, the ratio of the foam height at 50 ° C. and 20 ° C. represented by the above formula is set to 0 to 3%. By maintaining such a ratio of the foam height within a predetermined range, the resinification reaction (urethanization reaction), the foaming reaction, and the curing reaction (crosslinking reaction) during the production of the flexible polyurethane foam are made appropriate. In addition, cell formation can be performed in a stable state, and cell closure can be suppressed. As a result, cracks can be suppressed while maintaining the curability and shrinkage of the foam.

  In the flexible polyurethane foam according to the second aspect of the present invention, the blending amount of water is set to 5 to 8 parts by mass per 100 parts by mass of the polyols, so that the progress of the foaming reaction is suppressed to a predetermined range. Therefore, the effect of the invention according to claim 1 can be improved.

  In the flexible polyurethane foam of the invention according to claim 3, since the blending amount of the foam stabilizer is set to 1.7 to 1.9 parts by mass per 100 parts by mass of the polyols, as the foaming reaction proceeds. The foam regulating action is uniformly expressed within a certain range. For this reason, the effect of the invention which concerns on Claim 1 or Claim 2 can be improved more.

  In the flexible polyurethane foam of the invention according to claim 4, the catalyst is constituted by a resinification catalyst and a foaming catalyst, and the blending amount of the resinization catalyst is 0.15 to 0.25 parts by mass per 100 parts by mass of polyols. The blending amount of the foaming catalyst is set to 0.05 to 0.2 parts by mass per 100 parts by mass of the polyols. For this reason, resinification reaction and foaming reaction are controlled, resination and foaming proceed smoothly, and cells are formed stably. Therefore, the effect of the invention according to any one of claims 1 to 3 can be further improved.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments embodying the present invention will be described in detail.
The flexible polyurethane foam of the present embodiment (hereinafter also simply referred to as a foam) is made of a raw material of a flexible polyurethane foam containing polyols, polyisocyanates, a catalyst, water as a foaming agent, and a foam stabilizer. It is obtained by heating, heating, reaction, foaming and curing, and an apparent density is 20 to 30 kg / m ³ . The reason for heating to a high temperature of 120 to 150 ° C. is to obtain a so-called hot foam suitable as a sheet material for automobiles. If this temperature is less than 120 ° C, the reaction, foaming and curing of the foam material tends to be insufficient, and if it exceeds 150 ° C, the foam tends to discolor, which is not preferable. The apparent density is a value based on the specification of JIS K 7222: 1999. When the apparent density is 20 to 30 kg / m ³ , the foam is lightweight and rich in cushioning properties. Here, the soft polyurethane foam means a foam having an open-cell structure and having flexibility and resilience.

  Furthermore, the ratio of the foam height shown below is set to 0 to 3%. By setting the ratio of the foam height to this range, the formation of cells can be stabilized and the formation of cracks can be suppressed. This crack is unlikely to occur when the foam is formed of a flat surface such as a rectangular parallelepiped, and when it has an uneven shape, the formation of cells is inhibited at the uneven portion and is likely to occur. Therefore, it is effective to set the foam height ratio in the above range when the foam has an uneven shape.

Ratio of foaming height = {[(foaming height at 50 ° C.) − (Foaming height at 20 ° C.)] / (Foaming height at 20 ° C.)} × 100
An apparatus for producing a flexible polyurethane foam for obtaining this foam height ratio will be described. As shown in FIG. 1, the foaming container 10 is formed in a bottomed rectangular tube shape, around which water 11 as a heat medium is accommodated to set the inside of the foaming container 10 at a predetermined temperature. 12 is provided. Then, in a state where the temperature of the water 11 is set to 20 ° C. or 50 ° C., a certain amount of the raw material of the flexible polyurethane foam is put into the foaming container 10, and the height H of each foam 13 obtained by foaming (respectively H Height H ₁ or H ₂ ) is measured. Therefore, using these H ₁ or H ₂ , the ratio of the foam height is calculated from the above formula.

Next, the raw materials for the flexible polyurethane foam will be described in order.
As the polyols, polyether polyols or polyester polyols are used. Examples of polyether polyols include polypropylene glycol, polytetramethylene glycol, modified products thereof, and compounds obtained by adding alkylene oxide to glycerin. Specific examples include triols obtained by addition polymerization of propylene oxide to glycerin and addition polymerization of ethylene oxide, and diols obtained by addition polymerization of propylene oxide to dipropylene glycol and addition polymerization of ethylene oxide. Since these polyether polyols have a primary hydroxyl group at the terminal, they are preferable in that they are highly reactive with polyisocyanates and do not hydrolyze like polyester polyols. The average molecular weight of the polyether polyol is preferably 2000 to 6000. When the average molecular weight is less than 2000, the stability of the obtained foam at the time of molding decreases, and when it exceeds 6000, the resilience of the foam increases and the cushioning property decreases. This polyether polyol can change the number of functional groups and the hydroxyl group value of the hydroxyl group by adjusting the kind of raw material component, the molecular weight, the degree of condensation, and the like.

  As polyester polyols, in addition to condensation polyester polyols obtained by reacting polycarboxylic acids such as adipic acid and phthalic acid with polyols such as ethylene glycol, diethylene glycol, propylene glycol and glycerin, lactone polyester polyols and polycarbonate systems A polyol is mentioned. These polyols can change the number of hydroxyl groups and the hydroxyl value by adjusting the kind of raw material components, the molecular weight, the degree of condensation, and the like.

  The hydroxyl value of the polyols is preferably less than 250 (mgKOH / g), more preferably 50 to 70 (mgKOH / g). By using polyols having such a hydroxyl value, it is possible to obtain a polyurethane foam that is excellent in reactivity with polyisocyanates and is appropriately crosslinked. When the hydroxyl value of polyols is 250 (mgKOH / g) or more, the crosslinking density becomes too high and the foam tends to be hard. On the other hand, when the hydroxyl value is less than 50 (mgKOH / g), the hydroxyl value becomes too small, and the crosslinking density of the polyurethane foam tends to be low, and the strength of the foam tends to decrease.

  Subsequently, the polyisocyanates to be reacted with the above polyols are compounds having a plurality of isocyanate groups, specifically, tolylene diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI), and 1,5-naphthalene. Aromatic polyisocyanates such as diisocyanate (NDI), triphenylmethane triisocyanate, xylylene diisocyanate (XDI), alicyclic polyisocyanates such as isophorone diisocyanate (IPDI), dicyclohexylmethane diisocyanate, hexamethylene diisocyanate (HDI), etc. Aliphatic polyisocyanates, or modified isocyanates such as free isocyanate prepolymers and carbodiimide-modified polyisocyanates by reaction of these with polyols S, more mixtures polyisocyanate is used.

  Of these, tolylene diisocyanate and derivatives thereof, 4,4-diphenylmethane diisocyanate and derivatives thereof are preferable, and these can also be used as a mixture. Examples of tolylene diisocyanate and derivatives thereof include a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, and an isocyanate prepolymer derivative of tolylene diisocyanate. Examples of 4,4-diphenylmethane diisocyanate and derivatives thereof include a mixture of 4,4-diphenylmethane diisocyanate and its polymer polyphenylpolymethylene diisocyanate and diphenylmethane diisocyanate derivatives having terminal isocyanate groups.

  The isocyanate index (isocyanate index) of the polyisocyanates is preferably in the range of 80 to 95 in order to suppress the curing reaction of the foam material and suppress the shrinkage of the foam. When the isocyanate index is less than 80, the curability is insufficient, and when it exceeds 95, the curing reaction proceeds excessively and the shrinkage of the foam tends to deteriorate. Here, the isocyanate index is a percentage of the ratio of the number of equivalents of isocyanate groups of polyisocyanates to the number of equivalents of active hydrogen such as polyols and water as a blowing agent.

  Next, the catalyst promotes the resinification catalyst that promotes the resinification reaction (urethanization reaction) between polyols and polyisocyanates, and the foaming reaction between the polyisocyanates and water, effectively generating carbon dioxide. There are foaming catalysts used for improving the fluidity of raw materials and the dimensional stability of foams, and it is preferable to use these in combination. Specific examples of the resinification catalyst include triethylenediamine (TEDA), a mixture of triethylenediamine and polypropylene glycol, N, N, N ', N'-tetramethylethylenediamine, N, N, N', N'-tetramethylpropylene Diamine, N, N, N ′, N ″, N ″ -pentamethyl- (3-aminopropyl) ethylenediamine, N, N, N ′, N ″, N ″ -pentamethyldipropylenetriamine, N, N, N ′ , N′-tetramethylguanidine, tertiary amines such as 135-tris (N, N-dimethylaminopropyl) hexahydro-S-triazine, 1-methylimidazole, 1,2-dimethylimidazole, 1-isobutyl-2- Imidazoles such as methylimidazole, other N, N, N ′, N′-tetramethylhexamethylenediamine, N-methyl-N ′-(2 -Dimethylaminoethyl) piperazine, N, N'-dimethylpiperazine, N-methylpiperazine, N-methylmorpholine, N-ethylmorpholine and the like.

  Specific examples of the foaming catalyst include bis (2-dimethylaminoethyl) ether, N, N, N ′, N ″, N ″ -pentamethyldiethylenetriamine, N, N, N ′, N ″, N ′ ″. , N ′ ″-hexamethyltriethylenetetramine and the like.

  The blending amount of the resinification catalyst is preferably 0.15 to 0.25 parts by mass per 100 parts by mass of polyols. If the blending amount is less than 0.15 parts by mass, the resinification reaction is not sufficiently promoted, and physical properties such as the hardness of the foam are insufficient. Progression is promoted excessively and cell formation tends to be non-uniform. Moreover, it is preferable that the compounding quantity of a foaming catalyst is 0.05-0.2 mass part per 100 mass parts of polyols. When the blending amount is less than 0.05 parts by mass, the foaming reaction is not sufficiently promoted, and when it exceeds 0.2 parts by mass, the foaming reaction proceeds too much and foaming is locally excessive. It is not preferable.

  The foaming agent is for obtaining a flexible polyurethane foam by foaming a raw material of the flexible polyurethane foam, and includes at least water as a chemical foaming agent. Water mainly reacts with polyisocyanates (foaming reaction) to generate carbon dioxide gas. As the foaming agent, other chemical foaming agents or physical foaming agents can be used in combination with water. Examples of the chemical foaming agent include inorganic acids such as organic acids and boric acid, alkali metal carbonates, cyclic carbonates, dialkyl carbonates, and the like. These chemical blowing agents generate gas by reaction with foam raw material components or decomposition by heating. On the other hand, as physical blowing agents, hydrocarbons such as pentane and cyclopentane, hydrochlorofluorocarbons such as HCFC-22 and 141b, hydrofluorocarbons such as HFC-245 and 356, air, nitrogen and carbon dioxide gas Gas such as (carbon dioxide).

  The blending amount of water as a blowing agent is preferably 5 to 8 parts by mass per 100 parts by mass of polyols. When the blending amount of water is less than 5 parts by mass, the foaming reaction is insufficient and it becomes difficult to obtain a low-density foam, and the cushioning property is easily impaired. The foaming reaction becomes excessive and the formation of the cells becomes non-uniform so that the cells are likely to be partially closed.

  As the foam stabilizer, any of those usually blended in the raw material of the flexible polyurethane foam can be used. For example, nonionic surfactants such as organosiloxane-polyoxyalkylene copolymer, silicone-grease copolymer, etc. Or a mixture thereof. The blending amount of the foam stabilizer is preferably 1.7 to 1.9 parts by mass per 100 parts by mass of polyols. When the blending amount is less than 1.7 parts by mass, the foam regulating action is not sufficiently exerted, and the cell formation tends to be biased. When the blending amount exceeds 1.9 parts by mass, an excessive amount of the foam stabilizer may cause the foaming process. Stabilization is likely to be hindered.

  In addition, the raw material of the flexible polyurethane may contain a cell opener such as a polyalkylene oxide polyol, a flame retardant such as a condensed phosphate ester, an antioxidant, a plasticizer, an ultraviolet absorber, a colorant and the like.

  When performing the urethanization reaction between the polyols and polyisocyanates, a molding method using a molding die, the foam material is discharged onto a belt conveyor, naturally foamed at room temperature and atmospheric pressure, and cured. Any slab molding method or the like is employed. In the urethanization reaction, a one-shot method or a prepolymer method is employed. The one-shot method is a method in which polyols and polyisocyanates are directly reacted. The prepolymer method is a method in which a part of a polyol and a polyisocyanate are reacted in advance to obtain a prepolymer having an isocyanate group or a hydroxyl group at a terminal, and the polyol or polyisocyanate is reacted therewith.

  Now, the operation of the present embodiment will be described. A flexible polyurethane foam as a hot foam is obtained by using a foam raw material containing polyols, polyisocyanates, a catalyst, water as a foaming agent, and a foam stabilizer at 120 to 150 ° C. Manufactured by reaction, foaming and curing. In this case, as an index for suppressing cracks in the foam, the ratio of the foam height when foaming is performed at 20 ° C. and 50 ° C. is set to 0 to 3%. For this reason, the resinification reaction, the foaming reaction, and the curing reaction during the production of the foam proceed in a well-balanced manner, and the cells are formed uniformly and appropriately. Therefore, the cell can be prevented from being closed, and the foam crack can be suppressed.

The effects exhibited by the above embodiment will be described collectively below.
-In the flexible polyurethane foam in embodiment, the ratio of the said foaming height is set to 0 to 3%. By maintaining such a ratio of the foam height within a predetermined range, it is possible to moderate the resinification reaction, foaming reaction, and curing reaction during the production of the flexible polyurethane foam, and smooth cell formation is achieved. It is possible to suppress cell closure. As a result, cracks can be suppressed while maintaining curability and shrinkage.

-Moreover, foaming reaction can be suppressed to a predetermined range by setting the compounding quantity of water to 5-8 mass parts per 100 mass parts of polyols.
Furthermore, by setting the blending amount of the foam stabilizer to 1.7 to 1.9 parts by mass per 100 parts by mass of the polyols, the foam stabilizer can be uniformly performed within a certain range.

  As a catalyst, a resination catalyst and a foaming catalyst are used in combination, the blending amount of the resinating catalyst is 0.15 to 0.25 parts by mass per 100 parts by mass of polyols, and the blending amount of the foaming catalyst is 100 masses of polyols By setting to 0.05-0.2 mass part per part, resinification reaction and foaming reaction can be controlled, and resinification and foaming can be advanced smoothly.

Hereinafter, the embodiment will be described more specifically with reference to examples and comparative examples.
(Examples 1-7 and Comparative Examples 1-4)
As raw materials for the flexible polyurethane foam, polyether polyol, tolylene diisocyanate (TDI), water as a foaming agent, resinification catalyst, foaming catalyst and foam stabilizer were set as shown in Table 1. The contents of each component will be described below.

Polyether polyol: Polyether polyol obtained by addition polymerization of propylene oxide to glycerin and further addition polymerization of ethylene oxide, a polymer having an average molecular weight of 3000 and a hydroxyl value of 60 mgKOH / g, and having a polyethylene oxide unit of 12 mol% TDI : Tolylene diisocyanate (a mixture of 80% by mass of 2,4-tolylene diisocyanate and 20% by mass of 2,6-tolylene diisocyanate), T-80 manufactured by Nippon Polyurethane Industry Co., Ltd.
Resinization catalyst: 1-isobutyl-2-methylimidazole and N-methylmorpholine were used in a ratio of 1: 2.

Foaming catalyst: bis (2-dimethylaminoethyl) ether Foam stabilizer: Silicone foam stabilizer, manufactured by Toray Dow Silicone, SRX294A
After mixing the polyether polyol and TDI, the mixture was stirred for 3 seconds at 1800 rpm with a three-blade stirrer, and 300 g was put into a foaming container (inner volume 20 L) made of an aluminum square tube to foam. To produce a flexible polyurethane foam. In this case, the temperature of the raw material was set to 20 ° C., the temperature of the foaming container was set to 20 ° C. and 50 ° C., the foam height was calculated from the respective foam heights, and the ratio of the foam height was calculated using the following formula. .

Foam height ratio = {[(H ₁ ) − (H ₂ )] / (H ₂ )} × 100
However, H ₁ represents the foam height at 50 ° C., H ₂ represents the foam height at 20 ° C..
About the obtained flexible polyurethane foam, the density, crack, curability and shrinkage were measured by the methods shown below, and the results are shown in Table 1.
(density)
It is an apparent density (kg / m ³ ) measured in accordance with JIS K 7222: 1999.
(crack)
For soft polyurethane foam, visually confirm the state of the cell, ○ in the case of 2 or less cavities less than 1 cm square in the cross section of the foam, △ in the case of 3-5 cavities less than 1 cm square, When there were 1 or more cavities of 1 cm square or more, it was evaluated as x.
(Curable)
The measurement was made by holding the soft polyurethane foam by hand, and the case where the hardness was appropriate was evaluated as ◯, and the case where it was felt soft was evaluated as △.
(Shrinkage)
The shrinkage state when the flexible polyurethane foam was taken out from the container was observed, and the case where the shrinkage rate was less than 10% was evaluated as ◯, and the case where the shrinkage rate was 10% or more was evaluated as x.

  On the other hand, in Comparative Example 1, the foam height ratio was set to 7%, in Comparative Example 2 the foam height ratio was set to 5%, and in Comparative Examples 3 and 4, the foam height ratio was set to 4%. A flexible polyurethane foam was produced. Then, the density, cracks, curability and shrinkage were evaluated in the same manner as in Example 1, and the results are shown in Table 2.

表１に示したように、実施例１〜７では発泡高さ比が０〜３％の範囲に設定されていることから、クラック、硬化性及び収縮性がいずれも良好であった。これに対し、表２に示した結果より、比較例１〜４では発泡高さ比が４〜７％の範囲に設定されていることから、セルが崩れてクラックが不良であった。
（実施例８〜１４及び比較例５〜８）
軟質ポリウレタン発泡体の原料として、実施例８〜１４ではそれぞれ実施例１〜７と同じとし、比較例５〜８では比較例１〜４と同じとした。そして、それらの原料を発泡用容器に投入し、ホットフォームの反応温度である１４５℃に加熱して原料を反応、発泡及び硬化させ、軟質ポリウレタン発泡体を製造した。

As shown in Table 1, in Examples 1 to 7, since the foam height ratio was set in the range of 0 to 3%, the cracks, curability and shrinkage were all good. On the other hand, from the results shown in Table 2, in Comparative Examples 1 to 4, since the foam height ratio was set in the range of 4 to 7%, the cells collapsed and the cracks were poor.
(Examples 8-14 and Comparative Examples 5-8)
As raw materials for the flexible polyurethane foam, Examples 8 to 14 were the same as Examples 1 to 7, and Comparative Examples 5 to 8 were the same as Comparative Examples 1 to 4. Then, these raw materials were put into a foaming container and heated to 145 ° C., which is a hot foam reaction temperature, to react, foam and harden the raw materials to produce a flexible polyurethane foam.

  As a result, in the foams of Examples 8 to 14, the cells were formed satisfactorily, no crack was generated, and the curability and shrinkage were also good. On the other hand, in Comparative Examples 5 to 8, the formation of cells was uneven and locally closed, resulting in cracks. From the above facts, it became clear that the foam height ratio between the foam height at 50 ° C. and the foam height at 20 ° C. can be used as an index for cracks in the production of hot foam.

In addition, this embodiment can also be changed and embodied as follows.
・ As the ratio of foam height, obtain the foam height when foamed in the range of about 60-80 ° C. in addition to 50 ° C., calculate the ratio of the foam height to the foam height at 20 ° C. It is also possible to manufacture hot foams taking into account the ratio.

  -Further, as the ratio of foam height, the foam height when foamed at 15 ° C., 25 ° C., etc. in addition to 20 ° C. is calculated, and the ratio between the foam height and the foam height at 50 ° C. is calculated, It is also possible to manufacture hot foams taking into account the ratio.

A crosslinking agent such as glycerin or trimethylolpropane can be blended as the foam material, and the curability of the foam material can be adjusted by the blending amount.
Furthermore, the technical idea grasped from the embodiment will be described below.

  -The flexible polyurethane foam according to any one of claims 1 to 4, which has an uneven shape. When configured in this manner, the effects of the invention according to any one of claims 1 to 4 can be effectively exhibited.

  The flexible polyurethane foam according to claim 4, wherein the resinification catalyst contains a morpholine compound. When comprised in this way, the sclerosis | hardenability in the foam surface can be improved in the final stage of resinification.

Explanatory drawing which shows typically the cross section of the apparatus for manufacturing a flexible polyurethane foam.

Explanation of symbols

  13: Foam.

Claims (4)

It is obtained by reacting, foaming and curing a raw material of a flexible polyurethane foam containing polyols, polyisocyanates, a catalyst, water as a foaming agent and a foam stabilizer at 120 to 150 ° C., and an apparent density of 20 to 30 kg / m. ^3. A soft polyurethane foam, characterized in that the foam height ratio shown below is 0 to 3%.
Ratio of foaming height = {[(foaming height at 50 ° C.) − (Foaming height at 20 ° C.)] / (Foaming height at 20 ° C.)} × 100 2. The flexible polyurethane foam according to claim 1, wherein the amount of water is 5 to 8 parts by mass per 100 parts by mass of polyols. 3. The flexible polyurethane foam according to claim 1, wherein the blending amount of the foam stabilizer is 1.7 to 1.9 parts by mass per 100 parts by mass of polyols. The catalyst is composed of a resinization catalyst and a foaming catalyst. The blending amount of the resinating catalyst is 0.15 to 0.25 parts by mass per 100 parts by mass of polyols, and the blending amount of the foaming catalyst is 100 polyols. It is 0.05-0.2 mass part per mass part, The flexible polyurethane foam as described in any one of Claims 1-3 characterized by the above-mentioned.

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