JP3516874B2 - Method for producing polyurethane foam and polishing sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a polyurethane foam having uniform fine cells. The microcellular polyurethane foam of the present invention can be suitably used as a polishing material suitable for polishing resin, glass or lenses, silicon for manufacturing semiconductors and the like, electronic substrates, optical substrates and the like. The present invention also relates to a polishing sheet suitable as an abrasive material, which is obtained by appropriately cutting the polyurethane foam obtained by the above-mentioned manufacturing method.

[0002]

2. Description of the Related Art As a technique for producing a polyurethane foam, a low boiling point organic solvent such as CFC or methylene chloride is added to and dispersed in a foam-forming raw material composition, and the polymer is foamed by vaporization by heat of polymerization. A method is known in which water is added to and dispersed in a foam-forming raw material composition, and the polymer is foamed by carbon dioxide gas generated by the reaction of an isocyanate group and water. The foam obtained by these methods has a lower limit of the average cell diameter (cell diameter) of 100 μm, and it is difficult to obtain a foam having finer and more uniform cells.

The following methods are known as a method for producing a polyurethane foam having fine cells. (1) A solvent-soluble fine particle is dispersed in a polyurethane polymer and molded into a predetermined shape, and then the molded article is immersed in a solvent in which the fine particle dissolves but the polyurethane polymer does not dissolve to remove the fine particle to remove the porous polyurethane. A method of forming a resin, that is, a foam. (2) A method of dispersing the micro hollow foam in the polyurethane resin forming raw material composition.

[0004]

However, according to the above method (1), a large amount of solvent is required, and it is necessary to treat the solvent containing the fine particle forming material, which is costly. Moreover, the obtained foam has only open cells, and cannot be used for applications requiring rigidity, thus limiting the applications. Further, an elution step and a step of drying the solvent are also required, and there is a problem that it takes a long time to form a molded product having a large thickness.

On the other hand, in the method (2), it is difficult to produce a uniform foam since the hollow microfoam has a strong tendency to float up in the polyurethane reaction stock solution due to the difference in density. Is relatively expensive,
Further, there is a problem that the material of the minute hollow foam remains in the foam and the blade is damaged when the foam is cut. Moreover, the hollow fine particles are easily scattered, and a large amount of cost is required for equipment for maintaining the working environment.

The object of the present invention is to have uniform fine bubbles without using a chemically reactive foaming agent such as water, a vaporizing expandable foaming agent such as freon, a micro hollow foam, a solvent-soluble substance or the like different substances. It is to provide a method for producing a polyurethane foam.

[0007]

Means for Solving the Problems The present inventor has found that an isocyanate prepolymer which is a raw material liquid for producing polyurethane is used.
Included a surfactant to the first component comprising these liquid by stirring strongly this to the presence of a non-reactive gas becomes bubbles dispersion comprising bubbles of fine non-reactive gas, which in the chain A polyurethane foam having a uniform fine cell structure can be obtained by adding an extender and mixing the mixture to carry out a polymerization reaction. Further, an isocyanate prepolymer is included.
If the first component contains a surfactant, the reaction stock solution (first
By vigorously stirring one component and the chain extender) in the presence of a non-reactive gas, these liquids become a bubble dispersion liquid containing fine non-reactive gas bubbles, and are polymerized and cured as they are to form a uniform fine particle. It has been found that a polyurethane foam having a cell structure can be obtained, and the present invention has been completed.

The invention described in claim 1 of the present application is an isocyanate.
The silicone-based nonionic surfactant that does not have a hydroxyl group is used as the first component containing the polymer prepolymer.
5 to 50% by weight is added to a foam, and the first component is stirred with a non-reactive gas to disperse the non-reactive gas as fine bubbles to form a bubble dispersion, and the bubble dispersion. To
The present invention relates to a method for producing a microcellular polyurethane foam , which comprises mixing and curing a chain extender having a molecular weight of 500 or less .

[0009] The invention according to the claims 2, Isoshi
The first component containing the anate prepolymer contains a silicon-based nonionic surfactant that does not have a hydroxyl group as a microcellular polyurethane
The first component and a chain extender having a molecular weight of 500 or less are mixed with a non-reactive gas and stirred to disperse the non-reactive gas as fine bubbles. The present invention relates to a method for producing a microcellular polyurethane foam, which is characterized in that it is made into a cell dispersion, and the cell dispersion is cured.

The term "non-reactive gas" as used herein means a gas composed of only a room temperature gas component that does not react with an isocyanate group or an active hydrogen group. Further, the gas may be positively fed into the liquid, or only the situation in which the gas is naturally involved during stirring. The fine bubbles mean those having an average bubble diameter of 80 μm or less. According to the method of the present invention, it is possible to manufacture an average particle size of about 10 μm. The bubble diameter can be set and controlled by appropriately selecting and adjusting conditions such as the type of surfactant used, the amount added, stirring conditions, and the viscosity of the raw materials used. The density of the obtained foam is preferably about 0.6 to 0.9, and the hardness (ASKER C) of the foam is preferably 85 to 100. Especially, the foam for polishing has a hardness of 9
It is preferably 0 to 95.

The stirring time varies depending on the performance of the stirrer, the viscosity of the reaction stock solution forming the polyurethane foam, etc.
It is at least 30 seconds, more preferably about 1 minute to prepare a stable bubble dispersion, and stirring is possible as long as fluidity is secured. This stirring time needs to be longer than the stirring time for producing a normal polyurethane foam. The diameter of the bubbles can be adjusted by the manufacturing conditions such as the type of surfactant to be added, the amount added, and the stirring time.

In any of the above-mentioned methods for producing a microcellular polyurethane foam of the present invention, it is preferable that in any case, the method further comprises the step of passing the cellular dispersion through a sieve mesh. In the invention described in claim 1, the first component, which is the air bubble dispersion liquid, may be passed through the sieve mesh, or may be passed after being mixed with the chain extender . Further, in the invention according to claim 2, regardless of the order of mixing the first component, the chain extender and the non-reactive gas, the bubble dispersion is formed in a state where the first component and the chain extender are mixed. Good.

By passing through the sieve mesh, large bubbles generated during stirring are ruptured and disappeared, and it becomes possible to obtain a fine-cell polyurethane foam having a higher degree of cell uniformity.

In the present invention, isocyanate prepoly
By adding a silicon-based nonionic surfactant having no hydroxyl group to the first component containing a mer, a polyurethane foam having excellent physical properties and fine and uniform cells (cells) can be obtained.

The surfactant used in the present invention is a silicon-based nonionic surfactant having no hydroxyl group, and the use of such a surfactant does not impair the physical properties of polyurethane and causes bubbles. A fine and uniform polyurethane foam can be stably obtained.

The above-mentioned surface active agent is a fine air bubble powder to be synthesized.
Add 5 to 50% by weight to polyurethane foam
To do. If it is less than 5% by weight, a fine foamed cell cannot be obtained, and if it exceeds 50% by weight, the physical properties of the finely foamed polyurethane foam are deteriorated.

A surfactant is added to an isocyanate prepolymer containing an isocyanate group to form a chain as a curing agent.
In the method of adding an extender and mixing with a non-reactive gas, stirring, and dispersing the non-reactive gas as fine bubbles to form a bubble dispersion, and curing the bubble dispersion, the isocyanate of the isocyanate prepolymer is The group preferably contains an isocyanate group derived from an aliphatic isocyanate compound.

Isocyanate groups derived from an aliphatic isocyanate compound are less reactive than isocyanate groups derived from an aromatic isocyanate compound, and therefore, after being mixed with a chain extender , they are cured by reaction and the reaction mixture flows. It takes a long time to stop. Therefore, the dispersion time,
There is an effect that it is possible to secure the time required for pouring into a predetermined mold and molding, and further the time required for the step of passing the air bubble dispersion liquid through the screen. An isocyanate group derived from an aromatic polyisocyanate compound may coexist in this isocyanate prepolymer.

The aliphatic isocyanate compound is
It is sufficient that the isocyanate group is not directly bonded to the aromatic ring, and it does not matter whether or not the isocyanate compound has an aromatic ring.

[0020] The present invention is a polyurethane foam obtained by the production method described above, also relates to abrasive sheet using a foam, the contact surface of the polishing sheet and the object to be polished and polishing dust and abrasive surface of the sheet It is characterized in that a groove having a function of escaping from the outside is provided. The seat is
The polyurethane foam may be produced by pouring the reaction components into a mold having the same cavity as the intended sheet thickness. Alternatively, a thick block-like foam may be produced and cut into a predetermined thickness. It may be manufactured.

The shape of the groove is not particularly limited, but the cross section may be rectangular, triangular, U-shaped, semicircular, or the like, and may have a cross-sectional area through which fine powder passes. The grooves are arranged on the sheet surface in a concentric circle shape, a lattice shape, or the like.

[0022]

Oite to the embodiment of the present invention, the polyurethane
Isocyanate prepolymers known in the field of
Can be used indefinitely . When the isocyanate prepolymer is used, the resulting microcellular polyurethane foam has excellent physical properties.

Specific examples of the organic diisocyanate that can be used as a raw material for the isocyanate prepolymer in the present invention include the following compounds.

Aromatic diisocyanate compound, 4,4'-diphenylmethane diisocyanate, 2,
Aliphatic diisocyanate compounds such as 4'-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, naphthalene diisocyanate, 1,4-phenylene diisocyanate, ethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 1 , Aliphatic diisocyanates such as 6-hexamethylene diisocyanate (HDI), hydrogenated 4,4'-diphenylmethane diisocyanate (HMDI, trade name Hiren-W, manufactured by Huls),
1,4-Cyclohexane diisocyanate (CHD
I), isophorone diisocyanate (IPDI), hydrogenated m-xylylene diisocyanate (HXDI),
The above compounds such as alicyclic diisocyanates such as norbornane diisocyanate, xylylene diisocyanate (XDI), and tetramethyl xylylene diisocyanate (TMXDI) may be used alone or in combination.

In addition to the above diisocyanate compound,
A trifunctional or higher polyfunctional polyisocyanate compound can also be used. As the polyfunctional isocyanate compound,
A series of diisocyanate adduct compounds are commercially available as Desmodur-N (Bayer Co.) and trade name Duranate (manufactured by Asahi Kasei Corporation). These trifunctional or higher functional polyisocyanate compounds are preferably used by adding them to the diisocyanate compound because they are easily gelated during the prepolymer synthesis when used alone.

In the present invention, the first component is used.
Socia Prepolymer is the reaction product of a polyol compound to be described later with the isocyanate compound, Lee
Isocyanate group of isocyanate compound (NCO) and port
Equivalent ratio of active hydrogen groups (H ^* ) of liol compound NCO /
H ^* is an oligomer of the NCO group-terminated produced by heating the reaction in the range of 1.6 to 2.6. Equivalent ratio NCO / H ^*
Is preferably in the range of 1.8 to 2.2 . The use of commercially available isocyanate prepolymers is also suitable.

The isocyanate group in the invention described in claim 4 can be obtained by using the above-mentioned aliphatic isocyanate compound as at least one component of the prepolymer-forming component. If the unreacted NCO group at the terminal is derived from an aliphatic isocyanate compound, an aromatic diisocyanate may be used as a prepolymer synthesis component.

Used in the synthesis of isocyanate prepolymers
The polyol compound to be used is at least 2 or more active hydrogen.
It is an organic compound having atoms .

The active hydrogen group possessed by the polyol compound is a functional group containing hydrogen which reacts with an isocyanate group and includes a hydroxyl group, a primary or secondary amino group, a thiol group (S
H) etc. are illustrated.

The polyol compound is an oligomer having a molecular weight of about 500 to 10,000 as determined by the terminal group quantitative method,
Specifically, the following are exemplified.

1) Polyether polyol As the polyether polyol, propylene oxide is added to one or more polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerin and trimethylolpropane. Polyoxypropylene polyols obtained, polyoxyethylene polyols obtained by adding ethylene oxide, butylene oxide, polyols obtained by adding styrene oxide, etc., and tetrahydrofuran by ring-opening polymerization to the polyhydric alcohol The polyoxytetramethylene polyols obtained by addition can be exemplified. Copolymers using two or more of the above cyclic ethers can also be used.

2) Polyester polyol As the polyester polyol, one of ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, cyclohexanedimethanol, glycerin, trimethylolpropane, pentaerythritol or other low molecular weight polyhydric alcohol is used. Or two or more and one or more of glutaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, terephthalic acid, isophthalic acid, dimer acid, hydrogenated dimer acid or other low molecular weight dicarboxylic acid or oligomer acid. Condensation polymer with, propiolactone, caprolactone,
Examples thereof include polyols such as ring-opening polymers of cyclic esters such as valerolactone.

3) Acrylic polyol In the acrylic copolymer, β-hydroxyethyl acrylate, β-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, β-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, acrylic acid Acrylic acid hydroxyalkyl esters such as β-hydroxypentyl or similar methacrylic acid hydroxyalkyl esters, and acrylic acid monoesters of polyhydric alcohols such as glycerin and trimethylolpropane or methacrylic acid monoesters thereof, N- Acrylic polyol having two or more hydroxyl groups in one molecule such as a copolymerizable monomer of a monoethylenically unsaturated monomer having a hydroxyl group such as methylolacrylamide or N-methylolmethacrylamide can be used.

As the acrylic polyol, telechelic acrylic polyol can also be used. Such a telechelic acrylic polyol is obtained by polymerizing an unsaturated monomer containing a (meth) acrylic acid ester in the presence of an alcohol compound with an organic peroxide-containing initiator in the presence of an organic sulfonic acid compound. It is a hydroxyl group-containing acrylic polymer. As the alcohol compound, aliphatic or alicyclic alcohols such as methanol and ethanol are preferable, and when the monofunctional alcohol is used as the alcohol compound, the active hydrogen group-containing acrylic polymer obtained is substantially bifunctional. When a diol is used as the above, the active hydrogen group-containing acrylic polymer becomes substantially tetrafunctional.

4) Other polyols Others, phenol resin polyols, epoxy polyols, polybutadiene polyols, polyisoprene polyols, polyester-polyether polyols, polymer polyols obtained by vinyl addition or dispersion of polymers such as acrylonitrile and styrene, and urea-dispersed polyols. , Carbonate polyols and the like can be used as the polyol of the present invention. It is also possible to use a polyol compound in which these polyol compounds are condensed with p-aminobenzoic acid and the active hydrogen groups are aromatic amino groups.

The chain extender used in the present invention has a molecular weight of 5
It is a compound of 00 or less. Specific examples of the chain extender include ethylene glycol, propylene glycol, 1,4
-Aliphatic low molecular weight glycols and triols represented by butanediol, trimethylolpropane, etc., aromatic diamines such as methylenebis-o-chloroaniline (MOCA), dicyclohexylmethane-4,4'-diamine, 1, Aromatic diols such as 4-bishydroxyethoxybenzene (Cuamine H (manufactured by Ihara Chemical Co., Ltd.)) and m-xylylenediol (manufactured by Mitsubishi Gas Chemical Co., Inc.)
Can be mentioned .

As the non-reactive gas used for forming the fine bubbles, those which are not flammable are preferable, and specifically, nitrogen, oxygen, carbon dioxide gas, a rare gas such as helium and argon, or a mixed gas thereof is preferable. Is used, and it is most preferable in terms of cost to use dry air to remove moisture.

The silicon-based nonionic surfactant used in the present invention is a finely divided product when the above-mentioned first component or mixture (first component and chain extender ) is stirred in the presence of a non-reactive gas. Any material that stably forms bubbles can be used without limitation.

In particular, isocyanate prepolymers and chain extension
A surfactant which is used as a foam stabilizer in the technical field of polyurethane in that it has good compatibility with long-form agents .
As described above, those which do not have an active hydrogen group such as a hydroxyl group which reacts with an isocyanate group are used. Specific examples thereof include silicone foam stabilizers SH-190, SH-192 (manufactured by Toray Dow Corning Silicon Co., Ltd.), L-5340 (Nippon Unicar Co., Ltd.) and the like.

In the present invention, the non-reactive gas is made into fine bubbles to form the first component or mixture ( chain extension with the first component).
As a stirring device for dispersing the agent , a known stirring device can be used without particular limitation, and specific examples thereof include a homogenizer, a dissolver, and a twin-screw planetary mixer (planetary mixer). However, since the stirring device that causes a strong temperature rise by stirring especially increases the reaction rate in the method of forming bubbles by mixing the first component and the chain extender as in the invention described in claim 2, Not preferable. Although the shape of the stirring blade of the stirring device is not particularly limited, the use of a whipper type stirring blade can obtain fine bubbles,
preferable.

In the invention described in claim 1,
Stirring for creating a bubble dispersion and stirring for adding and mixing a chain extender are performed, but the stirring in the latter stage may not be stirring for forming bubbles in particular, and a stirring device that does not involve large bubbles may be used. preferable. A planetary mixer is suitable as such a stirring device. Further, even if the same agitating device is used as the agitating device at the former stage and the latter stage, it is preferable that the agitating condition is adjusted such that the rotating speed of the agitating blade is adjusted if necessary. .

The conditions for producing the cell dispersion are not particularly limited as long as fine cells are formed and a cured product having a predetermined shape can be obtained, but the temperature is not lower than the melting point of the first component and the chain extender. It is necessary that the temperature is not higher than the temperature at which the curing reaction between the isocyanate group and the active hydrogen group does not proceed rapidly. Preferably 0 ° C to 140 ° C, more preferably 10 ° C.
~ 120 ° C. Incidentally, the curing reaction of isocyanate groups and the active hydrogen group is an exothermic reaction, isocyanate selecting
Types of bets prepolymer and chain extender, the degree of heating by the combinations are different. If the temperature rise of the system due to the heat of reaction is large, it is not preferable because the bubbles in the bubble dispersion expand.
When a reaction system having a small reaction heat or a reaction system having a large reaction heat is used, it is preferable to perform sufficient temperature control.

In the method for producing a polyurethane foam of the present invention, heating and post-curing the foam reacted by pouring the cell dispersion into a mold until the foam does not flow improves the physical properties of the foam. It is effective and extremely suitable. The conditions may be such that the air bubble dispersion liquid is poured into the mold and immediately placed in a heating oven for post cure. Even under such conditions, heat is not immediately transferred to the reaction components, and therefore the air bubble diameter does not increase. It is preferable to carry out the curing reaction at normal pressure because the bubble shape becomes stable.

In the present invention, a catalyst which accelerates the polyurethane reaction may be used. However, particularly as in the invention according to claim 2, an isocyanate prepolymer
When the first component containing a and the chain extender are mixed and stirred,
It is necessary to secure a sufficient stirring time for forming fine bubbles and a flowing time for pouring into a mold having a predetermined shape, and the type and addition amount of the catalyst are selected taking these into consideration.

The microcellular polyurethane foam of the present invention can be manufactured by a batch system in which each component is weighed and put in a container and stirred, or each component and a non-reactive gas are continuously fed to a stirrer. A continuous production method may be used in which a molded product is manufactured by supplying and stirring, and then sending out the air bubble dispersion liquid.

The sieve mesh used in the present invention is only required to remove relatively large air bubbles in the air bubble dispersion liquid, and in order to obtain a foam having a cell diameter of 50 μm or less, one having a finer mesh than 80 mesh should be used. Good. In the case of a continuous production system, it is preferable to provide the manufacturing apparatus in a strainer form.

In the present invention, other components may be added in addition to the polyurethane-forming raw material. Specific examples include catalysts or retarders for adjusting the curing reaction rate, and colorants such as dyes and pigments.

[0048]

EXAMPLES Examples of the present invention will be described below. (Example 1) Adiprene L-325 melted and adjusted in temperature to 80 ° C
(Isocyanate-terminated prepolymer, NCO = 9.25
% Uniroyal Co., Ltd.) 20 g of a silicone foam stabilizer SH-192 (manufactured by Toray Dow Corning Silicon Co., Ltd.) as a surfactant is added (first component) to a whipping mixer (stirring blade rotation speed = 3500 rpm). The mixture was stirred for about 5 minutes until it became creamy (meringue state) to obtain an air bubble dispersion liquid 1. This air bubble dispersion liquid 1 was transferred to a planetary mixer, and 26.6 g of Cuamine MT (methylenebis-o-chloroaniline, manufactured by Ihara Chemical Co., Ltd.) ( chain extender ) melted and kept at 120 ° C. was added to about 2 Mix for minutes. The mixed solution was cast into an open mold to be cured within a pot life having fluidity, and post-cured for 6 hours in an oven whose temperature was adjusted to 110 ° C. ± 10 ° C. to prepare a microcellular polyurethane foam. The surfactant used was 13.6 based on the total amount of the first component and the chain extender.
% By weight.

The resulting microcellular polyurethane foam is
It had uniform cells having a density of 0.75 g / cm ³ , a hardness (ASKER C) of 94, and an average cell diameter of 50 μm.

Example 2 Adiprene L-325 melted and temperature-controlled at 80 ° C.
(Isocyanate-terminated prepolymer, NCO = 9.25
% Uni-Royal Co., Ltd.) 100 g, and 20 g of a silicon foam stabilizer SH-192 (manufactured by Toray Dow Corning Silicon Co., Ltd.) as a surfactant is added to the premix (first composition).
In minutes), Kyuamin MT was melted and maintained at 120 ° C. Ltd. (methylenebis -o- chloroaniline, Ihara Chemical Corporation) (the chain extender) was 26.6g turned about until creamy (meringue state) The mixture was stirred for 2 minutes to obtain a bubble dispersion liquid 2. This cell-dispersed liquid 2 was cast into an open mold within a fluid working time, cured in an oven whose temperature was adjusted to 110 ° C ± 10 ° C, and post-cured for 6 hours to give a fine-celled polyurethane foam. Created. The amount of the surfactant used was 13.6 % by weight based on the total amount of the first component and the chain extender .

The microcellular polyurethane foam obtained is
It had uniform cells with a density of 0.75 g / cm ³ , a hardness (ASKER C) of 94, and an average cell diameter of 30 μm.

Example 3 Microcellular polyurethane in the same manner as in Example 2 except that the amount of the surfactant added was 50 g ( 28.3 % by weight based on the total amount of the first component and the chain extender ). A foam was created. The resulting microcellular polyurethane foam has a density of 0.75
It had g / cm ³ , hardness (ASKER C) of 94, and uniform cells having an average cell diameter of 30 μm.

Comparative Example 1 A polyurethane foam was prepared in the same manner as in Example 2 without adding a surfactant. The obtained polyurethane foam had a large average cell diameter by visual observation, was non-uniform, and was not a problem.

Comparative Example 2 Microcellular polyurethane in the same manner as in Example 2 except that the amount of the surfactant added was 5 g ( 3.8 % by weight based on the total amount of the first component and the chain extender ). A foam was created. The obtained polyurethane foam had an average cell diameter of 100 μm and could not be said to be fine cells.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-57-162772 (JP, A) JP-A-57-80418 (JP, A) JP-A-9-235344 (JP, A) JP-A-60- 96629 (JP, A) JP 56-88438 (JP, A) JP 55-155037 (JP, A) JP 1-222868 (JP, A) International Publication 98/017703 (WO, A1) (JP 58) Fields investigated (Int.Cl. ⁷ , DB name) C08G 18/00-18/87 C08J 9/00-9/42 B24D 3/00-18/00

Claims (7)

(57) [Claims] 1. A first containing an isocyanate prepolymer.
The silicone-based nonionic surfactant, which does not have a hydroxyl group, is added to the microcellular polyurethane foam in an amount of 5 to 50% by weight.
As described above, the first component is stirred with a non-reactive gas to disperse the non-reactive gas as fine bubbles to form a bubble dispersion, and the chain dispersion having a molecular weight of 500 or less is added to the bubble dispersion. ) Is mixed and cured, and a method for producing a microcellular polyurethane foam. 2. A first comprising an isocyanate prepolymer.
The silicone-based nonionic surfactant, which does not have a hydroxyl group, is added to the microcellular polyurethane foam in an amount of 5 to 50% by weight.
It was added to so that, first component and the molecular weight of 500 or less Kusarinobe
A method for producing a microcellular polyurethane foam, which comprises mixing a long medicine with a non-reactive gas and stirring the mixture to disperse the non-reactive gas as fine cells into a cell dispersion, and curing the cell dispersion. . 3. The method for producing a microcellular polyurethane foam according to claim 1, further comprising the step of passing the cell dispersion liquid through a sieve mesh. 4. The method for producing a microcellular polyurethane foam according to claim 1, wherein the isocyanate group of the isocyanate prepolymer contains an isocyanate group derived from an aliphatic isocyanate compound. 5. A microcellular polyurethane foam obtained by the method according to any one of claims 1 to 4. 6. A polishing sheet comprising the microcellular polyurethane foam according to claim 5. 7. The polishing sheet according to claim 6, wherein grooves are provided on the surface of the sheet.