JP2000109579A - Production of crystalline methacrylic resin and foam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a crystalline methacrylic resin having no limitation even on shape applicable to molding products, having excellent solvent resistance and mechanical characteristics, useful for illumination, or the like, by bringing a mixture of an isotactic and a syndiotactic methyl methacrylate-based polymers into contact with a high-pressure gas. SOLUTION: A mixture of an isotactic methyl methacrylate-based polymer and a syndiotactic methyl methacrylate-based polymer (the blending ratio is preferably 15/85-85/15 based on weight) is brought into contact with a high- pressure gas containing >=50 vol.% carbon dioxide (pressure is preferably >=1 MPa) to give a crystalline methacrylic resin. Preferably, after the mixture is brought into contact with the high-pressure gas, the mixture is expanded to produce a foam. Preferably, the foam comprises a fine foam having <=10 μm average cell diameter and 109 to 1015 cells/cm3 density of number of cells.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a crystalline methacrylic resin, a crystalline methacrylic resin foam, and a method for producing the same.

[0002]

2. Description of the Related Art Methacrylic resins have excellent properties of transparency, weather resistance and mechanical properties, so they are molded into various shapes, and are used for lighting, signboards, displays, building materials, foam materials, lenses, optical disks, etc. Widely used for optical equipment.

On the other hand, isotactic polymethyl methacrylate (hereinafter sometimes abbreviated as IPMMA) and syndiotactic polymethyl methacrylate (hereinafter SP)
It is widely known that, when mixed under appropriate conditions, a crystalline methacrylic resin called stereocomplex polymethyl methacrylate (hereinafter sometimes abbreviated as SCPMMA) can be obtained. I have. Various methods are known for obtaining such a crystalline methacrylic resin. For example, IPM
Mixing MA and SPMMA in a specific solvent such as toluene, acetone, DMF, or IPMMA and SP
There is a method of heat-treating a solid mixture of MMA (
The Chemical Society of Japan, "Experimental Chemistry Lecture 28 Polymer Synthesis" Maruzen
1992, p. 129, lines 10-13). In addition, IP
A method of dissolving each of MMA and SPMMA in methyl methacrylate and mixing to form SCPMMA, and then photopolymerizing methyl methacrylate to obtain a polymethylmethacrylate containing SCPMMA (Japanese Patent Publication No. 47-14834). Swell IPMMA with methyl methacrylate,
Matrix polymerization to form SCPMMA at the interface [H.Yau, and SIStupp, J.Polym.Sci., Polym.C
hem. Ed., 23, 813 (1985)]. It is known that the crystalline methacrylic resin thus obtained has better solvent resistance than non-crystalline methacrylic resin (JP-A-3-244651).

There is a field in which patterns and patterns are printed on the surface of methacrylic resin and applied. For example, in a light guide plate made of methacrylic resin, in order to improve the brightness of the light guide plate,
An appropriate dot pattern is provided on the surface of the light guide plate by screen printing. The printing in the method is usually
Organic solvents are used, but in such methods,
There is a case where a defect that the surface of the methacrylic resin is eroded by the solvent by printing and the printing becomes unclear is caused. Also, when manufacturing a decorative board coated with a methacrylic resin film in which various patterns are printed on the surface of a base material such as a metal material, the decorative board is usually used in a finishing process in order to remove an organic solvent used for printing with an alcoholic solvent. Clean the surface. In this case, cracks may occur in the methacrylic resin film due to residual stress in the coating step or in washing with the alcohol-based solvent.

In the above-mentioned applications, application of a crystalline methacrylic resin having excellent solvent resistance called SCPMMA as described above can be considered. However, in a conventionally known method for producing a crystalline methacrylic resin, In a method in which IPMMA and SPMMA are simply mixed in a special solvent such as above, the crystallinity of the obtained crystalline methacrylic resin is low, the solvent remains in the molded article, and the residual solvent deteriorates the solvent resistance. However, it has a drawback that it is difficult to use for the purpose of improving solvent resistance. In the method of heat-treating a mixture, IPMMA and S have relatively low molecular weight and high stereoregularity.
When PMMA is used, crystallization proceeds rapidly, but the raw material cost is high. On the other hand, when IPMMA and SPMMA having relatively high molecular weight and low stereoregularity are used, crystallization is difficult to proceed. In addition, the IP
After dissolving each of MMA and SPMMA in methyl methacrylate and mixing to form a stereo complex,
A molded product that can be applied is a method of obtaining a stereocomplex-containing polymethyl methacrylate by photopolymerizing a monomer or a method of swelling IPMMA with methyl methacrylate and polymerizing the matrix to form SCPMMA at the interface. Is limited.

[0006]

In view of such circumstances,
The present inventors do not have the above-mentioned drawbacks, that is, there is no limitation on the shape of a molded article that can be applied, and in a simple method, a high crystallinity, a crystalline methacrylic resin excellent in solvent resistance. As a result of intensive studies aimed at finding a manufacturing method,
When a mixture of IPMMA and SPMMA is treated under specific conditions, it has been found that a crystalline methacrylic resin satisfying all of the above objects can be obtained, and the present invention has been completed. When a mixture of IPMMA and SPMMA is treated under specific conditions and foamed, the mixture has excellent solvent resistance, an average cell diameter of about 10 μm or less, and a cell number density of 10 μm or less.
Found ^{that 9-10} crystalline methacrylic resin foam having excellent mechanical properties such as impact resistance and flexural strength with fine bubbles of ¹⁵ / cm ³ is obtained, and have completed the present invention.

[0007]

That is, the present invention provides a process for producing a crystalline methacrylic resin comprising a step of contacting a mixture of an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer with a high-pressure gas. Provide a way. Further, the present invention provides a crystalline methacrylic resin foam comprising a step of contacting a mixture of an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer with a high-pressure gas, and then foaming the mixture. It is to provide a manufacturing method. In addition, the present invention provides a cell having an average cell diameter of about 10 μm or less and a cell number density of 10 μm or less.
To provide a crystalline methacrylic resin foam characterized by having a fine bubbles ^{9-10 15} / cm ^3.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The isotactic methyl methacrylate-based polymer in the present invention is a polymer containing methyl methacrylate units as a main component, and the isotacticity of the methyl methacrylate unit chain is about 50 in triad display. %, Preferably at least about 80%. The isotactic methyl methacrylate polymer in the present invention may be a homopolymer of a methyl methacrylate monomer, or a monomer copolymerizable with a methyl methacrylate monomer and a methyl methacrylate monomer. It may be a copolymer with a monomer.

The monomer copolymerizable with the methyl methacrylate monomer is not particularly limited as long as it is a known monomer. Examples thereof include ethyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, and methacrylic acid. Methacrylic esters such as 2-ethylhexyl and 2-hydroxyethyl methacrylate, and acrylic esters such as methyl acrylate, ethyl acrylate, butyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate and 2-hydroxyethyl acrylate , Unsaturated acids such as methacrylic acid and acrylic acid, monofunctional monomers such as acrylonitrile, methacrylonitrile, and styrene; and polyfunctional monomers such as ethylene glycol dimethacrylate and trimethylpropane trimethacrylate. When these monomers are used for obtaining the isotactic methyl methacrylate-based polymer in the present invention, these may be used alone or in combination of two or more kinds together with the methyl methacrylate monomer.

The amount of the copolymerizable monomer is determined by the desired physical properties of the crystalline methacrylic resin, and is not particularly limited. , About 30% by weight or less, preferably 0 to about 20% by weight.
% By weight. If the amount of the copolymerizable monomer is too large, it may be difficult to form a crystalline methacrylic resin.

The isotactic methyl methacrylate polymer used in the present invention can be produced by a conventionally known method. For example, it can be obtained by anionic polymerization of a methyl methacrylate monomer using a Grignard reagent as a polymerization initiator (for example, JP-A-61-17).
No. 9210, JP-A-61-176617).

Another syndiotactic methyl methacrylate polymer applied to the present invention is a polymer having a methyl methacrylate unit as a main component, and a syndiotacticity of a methyl methacrylate unit chain is represented by a triad. Means about 50% or more, and if necessary, a copolymer with another copolymerizable monomer similar to the isotactic methyl methacrylate-based polymer.

The amount of the copolymerizable monomer is determined by the desired physical properties of the crystalline methacrylic resin, and is not particularly limited. It is less than 30% by weight, preferably 0 to about 20% by weight. If the amount of the copolymerizable monomer is too large, it may be difficult to form a crystalline methacrylic resin.

The syndiotactic methyl methacrylate polymer can be produced by a conventionally known method. For example, Japanese Patent Publication No. Hei 6-89054,
As described in JP-A-263412 and the like, it can be obtained by anionic polymerization of a methyl methacrylate monomer using an organic aluminum compound or an organic lanthanide complex as an initiator. It can also be obtained by polymerizing a methyl methacrylate monomer with a known radical polymerization initiator. Such radical polymerization initiators include, for example, organic peroxide initiators such as benzoyl peroxide, di-t-butyl peroxide, and t-butyl peroxy 2-ethylhexaate; 2,2 ′ azobisisobutyronitrile, 2, Azo-based initiators such as 2 'azobis (2,4-dimethylvaleronitrile); and known redox-based initiators in which a peroxide initiator is combined with a reducing compound such as an amine or a mercaptan as a main component. A photopolymerization initiator system in which benzoin, benzoin ethers, 1-hydrohexyl phenyl ketone, benzyldimethyl ketal, acylphosphenoxide, benzophenone, Michler's ketone, thioxanthone and the like are used in combination with a photosensitizer as required. Is mentioned.

One aspect of the present invention comprises a step of contacting a mixture of an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer with a high-pressure gas. The method for obtaining the mixture of the isotactic methyl methacrylate polymer and the syndiotactic methyl methacrylate polymer may be a known method, and is not particularly limited, but includes, for example, chloroform and methylene chloride. Or a method of melt-kneading an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer in a solvent capable of dissolving a crystalline methacrylic resin such as the above. Part of the mixture may be crystallized during the step of obtaining the mixture, and part of the solvent used in the step of obtaining the mixture may remain in the mixture.

The mixing ratio of the isotactic methyl methacrylate polymer and the syndiotactic methyl methacrylate polymer in the mixture is about 1: about 99% by weight.
~ About 99: about 1. Preferably about 10: about 90 to about 90: about 10, more preferably about 15: about 85 to about 8
5: about 15. When the amount of the isotactic methyl methacrylate-based polymer or the syndiotactic methyl methacrylate-based polymer is small, the solvent resistance of the crystalline methacrylic resin formed tends to be low. In the case of a foam, it tends to be difficult to obtain air bubbles having a fine cell diameter. The form of the mixture may be any form such as a film, a plate, or a desired molded product, and is not particularly limited.

In practicing the present invention, a mixture comprising the isotactic methyl methacrylate polymer and the syndiotactic methyl methacrylate polymer is then brought into contact with a high-pressure gas, so that the mixture contains a gas. The high-pressure gas to be brought into contact with the mixture may be a conventionally known gas, for example, carbon dioxide, nitrogen,
Examples thereof include argon, hydrogen, oxygen, butane, propane, and air. These may be used alone or as a mixture of two or more. In particular, it is inert to the resin to be treated, has high solubility in methyl methacrylate polymer,
It is preferable to use a high-pressure gas containing about 50% by volume or more of carbon dioxide from the viewpoint of easy handling.

The pressure of the high-pressure gas brought into contact with the mixture is
Although determined by the physical properties of the desired crystalline methacrylic resin, it is usually about 0.2 MPa or more, preferably about 1 MPa.
That is all. When the pressure is low, the melting point and the degree of crystallinity of the obtained crystalline methacrylic resin are low, and when the pressure is high, the melting point and the degree of crystallinity of the obtained crystalline methacrylic resin are high. Also, when the pressure is low, the cell diameter of the foam increases, whereas when the pressure increases, the cell diameter of the foam decreases. The upper limit of the high-pressure gas to be used is not particularly limited and is mainly determined by economics and the like, but is usually up to about 40 MPa.

The temperature of the high-pressure gas contacting the mixture also
It is not particularly limited and is determined by the physical properties of the desired crystalline methacrylic resin.
° C, preferably from about 0 to about 200 ° C. When the temperature is low, the melting point is low, and the cell diameter of the foam is small. If the temperature is too high, the resin may be decomposed, and the gas tends to be difficult to dissolve in the polymer.

The high-pressure gas brought into contact with the mixture is preferably in a supercritical state. The high-pressure gas being in a supercritical state means that the temperature and pressure of the high-pressure gas are at or above the critical point.In this state, changing the pressure changes the density, viscosity,
The diffusion coefficient and the like can be changed widely from a state close to a gas to a state close to a liquid. As is well known, the critical point of the high-pressure gas differs depending on the type of the high-pressure gas. For example, in carbon dioxide, the temperature is 304.2K, the pressure is 7.4 MPa,
For nitrogen, the temperature is 126.2K and the pressure is 3.4 MPa.
Even when two or more types of gases are mixed, there is a critical point according to the type of the mixed gas and the mixing ratio.

The time for contacting the mixture with a high pressure gas is also
Although it is not particularly limited and is determined by the shape of the methacrylic resin to be treated, it is usually about 0.1 second to about 7 seconds.
Days, preferably from about 30 seconds to about 12 hours. When the time is shorter than the above range, only a very thin portion of the surface layer is crystallized, and the upper limit of the processing time is not particularly limited, but even if the contact is performed for a long time, the effect corresponding to the time is lost, It is processed within the above range from normal operation efficiency.

The method of bringing the mixture into contact with the high-pressure gas is not particularly limited as long as the mixture can be brought into contact with the high-pressure gas in the high-pressure gas atmosphere. For example, a method of enclosing a high-pressure gas in a pressure vessel containing the mixture, a method of enclosing a gas at normal pressure in a pressure vessel containing the mixture, and then setting the inside of the vessel to a high-pressure gas atmosphere by an operation such as heating. Method and the like. The gas is contained in the mixture by contacting the mixture with a high pressure gas. As a specific method, for example, the mixture having an arbitrary shape such as a film shape, a plate shape, and a desired molded product shape is put into a pressure-resistant container, a gas is injected into the entire container, and the mixture is brought into contact with the high-pressure gas. A method of causing a gas to be contained in the mixture by introducing the mixture into the mixture, placing the mixture in a molten state in a pressure vessel, an extruder, an injection molding machine, or the like, injecting a high-pressure gas into the mixture, and introducing a gas into the mixture. And the like.

The mixture after the contact treatment with the high-pressure gas may then be taken out by reducing the pressure until it reaches normal pressure. At this time, an unfoamed transparent crystalline methacrylic resin can be obtained by slowly reducing the pressure to normal pressure. In addition, a crystalline methacrylic resin foam can be obtained by reducing the pressure at once.

The mixture obtained after the contact treatment with the high-pressure gas of the present invention thus obtained is simply IPMMA and SPM
The degree of crystallinity of the crystalline methacrylic resin obtained by a conventional method of heat-treating a mixture of MAs is at most about 2%, whereas it is usually 3% or more, usually 5%, preferably 1%.
It is a crystalline methacrylic resin having a high degree of crystallinity exceeding 0%, and is a crystalline methacrylic resin having extremely excellent solvent resistance. The crystalline methacrylic resin foam obtained by foaming the mixture has the above high crystallinity and usually has an average cell diameter of about 10 μm or less, usually about 5 μm or less.
μm or less, preferably about 2 μm or less, and a cell number density of 10
^It is a foam having fine cells of ⁹ to 10 ¹⁵ cells / cm ^3, a small average cell diameter of the foam, and a high cell number density, and the foam is formed of a conventionally known methyl methacrylate resin foam. By comparison, the foam has not only excellent solvent resistance but also excellent mechanical properties such as impact resistance and bending strength as compared with foams having the same expansion ratio.

[0025]

As described in detail above, according to the present invention, a crystalline methacrylic resin and a crystalline methacrylic resin foam having high crystallinity and excellent solvent resistance are provided by an extremely simple method. Lighting, signboards, displays, building materials, foam materials, lenses, optical discs, etc., because the shape of the molded article is not restricted as in the conventional methods and the methods introduced in the above. It can be applied to various fields such as optical equipment, and its industrial use value is extremely large.

[0026]

Hereinafter, the present invention will be described with reference to Examples.
The present invention is not limited to this. The physical property measurement and test methods used in the examples are as shown below.

Number average molecular weight: gel permeation
Chromatography (Waters 150-CV)
The molecular weight distribution of the test sample was measured at 40 ° C. using THF as a solvent. For the determination of the number average molecular weight, a calibration curve prepared from a polymethyl methacrylate standard sample was used.

Stereoregularity: 110 ° C. using a proton nuclear magnetic resonance spectrometer (XL-200 manufactured by Varian) using nitrobenzene-d5 as a solvent.
The nuclear magnetic resonance spectrum of the sample was measured.
For stereoregularity indication, isotactic methacrylic resin is isotacticity triad indication (mm)
I went in. Syndiotactic methacrylic resin is a syndiotactic triad (rr)
I went in.

Melting point and crystallinity: Scanning calorimeter (
The endothermic peak of the test sample was measured at a heating rate of 10 ° C./min by using SSC-5880II manufactured by Seiko Instruments Inc. The melting point (Tm) of the test sample was determined from the peak top of the absorption peak derived from the crystal. The change in heat of fusion (ΔH) was calculated and calculated from the area of the endothermic peak, and ΔH of 100% crystallinity was calculated to be 58 J / g.
(K. Konnecke, and G. Rehage, Makromol. Chem., 184, 2679-
2691 (1983)), the crystallinity of the test sample was calculated.

Solvent resistance test: When the test sample was in the form of a film, a 20 mm square film-like test piece was prepared and used for the solvent resistance test. Acetone was applied to the test surface, and the solvent resistance of the test piece was evaluated based on the presence or absence of craze after the application. The sample which was excellent in solvent resistance without crazing was evaluated as ○, and the sample which was crazed and poor in solvent resistance was evaluated as ×. When the test sample was a foam sample, acetone was dropped on the surface, dried at room temperature, and evaluated by the size of the trace of the drop. When the trace was small and inconspicuous and the solvent resistance was excellent, it was evaluated as ○. When the trace was slightly large, it was evaluated as Δ. When the trace was clearly recognized and the solvent resistance was poor, the cross was evaluated as X.

Average cell diameter and cell number density of bubbles: Scanning electron microscope (JSM-840A, manufactured by JEOL Ltd.)
Image processing software (Toyobo Co., Ltd., Image Analyzer V10LAB for
Statistical processing was performed using Windows 95), and the average value of the diameter of cells in the cross section of the foam was obtained. The value was defined as the average cell diameter D of bubbles in the test sample. In addition, the cell number density N
I asked. N = (n / A) ^3/2 / (1−4 / 3π (D / 2) ³ · (n / A) ^3/2 ) (where N is the cell number density, A is the area of the statistical processing area) , N
Is the number of cells in A, D is the average cell diameter)

Example 1 Isotactic methyl methacrylate polymer obtained by anionic polymerization (number average molecular weight 36,200, m
(m = 81%) 15 parts by weight, 15 parts by weight of a syndiotactic methyl methacrylate polymer (number average molecular weight = 55,800, rr = 57%) obtained by radical polymerization are dissolved in 70 parts by weight of methylene chloride. did. This solution was spread on a polyethylene terephthalate resin film,
Dried for 1 hour at room temperature and 1 hour in a vacuum dryer. A film-like mixture of an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer having an average thickness of 113 μm was obtained. When the crystallinity of this film was measured by a scanning calorimeter, the Tm was 12
2.8 ° C., crystallinity 0.7%. This film was placed in a pressure vessel, and the inside of the pressure vessel was filled with carbon dioxide at 30 ° C. and 5 MPa. After holding at 30 ° C. and 5 MPa for 6 hours, the pressure inside the pressure vessel was reduced to normal pressure at 5 MPa / h, and then the film was taken out. Tm and crystallinity of the obtained film were measured by a scanning calorimeter. Further, the obtained film was subjected to a solvent resistance test. Table 2 shows the results.

Examples 2 to 11 The temperature and pressure of the carbon dioxide filled in the pressure vessel were set to the values shown in Table 1, and the film was formed in the same manner as in Example 1 except that the film was kept in contact for the time shown in Table 1. Obtained. Tm and crystallinity of the obtained film were measured by a scanning calorimeter. Further, the obtained film was subjected to a solvent resistance test.
Table 2 shows the results.

Comparative Example 1 A film-like mixture obtained in the same manner as in Example 1 was used.
Tm and crystallinity were measured by a scanning calorimeter without any treatment (not put in a pressure vessel and not in contact with high-pressure carbon dioxide). Further, the obtained film was subjected to a solvent resistance test. Table 2 shows the results.

Comparative Examples 2 to 5 In Example 1, the polymer film mixture was placed in a pressure vessel, and the vessel was kept under a high-pressure carbon dioxide atmosphere.
Then, the same procedure as in Example 1 was repeated, except that the polymer film mixture was allowed to stand in an air oven under normal pressure for the time and temperature shown in Table 1 instead of performing the operation of returning to normal pressure. A film was obtained. The Tm and crystallinity of the obtained film were measured using a scanning calorimeter. Further, the obtained film was subjected to a solvent resistance test. Table 2 shows the results.

[0036]

[Table 1] * 1. About 0.1MPa

[0037]

[Table 2]

Example 12 An isotactic methyl methacrylate polymer obtained by anionic polymerization (number average molecular weight 36,200, m
(m = 81%) 10 parts by weight and 90 parts by weight of a syndiotactic methyl methacrylate-based polymer (number average molecular weight = 55,800, rr = 57%) obtained by radical polymerization are melt-kneaded with a single screw extruder. Then, a pellet of the mixture was obtained. The obtained mixture pellet was pressed at 220 ° C. to obtain a sheet having a thickness of about 1 mm. This sheet was placed in a pressure vessel, and the inside of the pressure vessel was filled with carbon dioxide at 40 ° C. and 20 MPa. After maintaining the carbon dioxide pressurizing condition of the same temperature and the same pressure for 5 hours, the carbon dioxide in the container was evacuated in 10 seconds, and the pressure in the container was reduced to normal pressure to obtain a foam. Using a scanning calorimeter, the Tm and crystallinity of the obtained foam were measured. Further, the average cell diameter D and the cell number density N of the cells of the foam were determined by image processing from the SEM photograph of the cross section of the obtained foam. Table 5 shows the results.

Examples 13 to 27 and Comparative Examples 6 to 9 In Example 12, the amounts of the isotactic methyl methacrylate polymer and the syndiotactic methyl methacrylate polymer were changed to 10 parts by weight and 90 parts by weight, respectively. The same operation as in Example 12 was carried out except that the parts by weight were changed to parts by weight shown in Table 3, and in each of the experimental examples and comparative examples, Table 3 was used.
In each experimental example and comparative example, except that the sheet having the thickness shown in Table 4 was obtained and the temperature and pressure of carbon dioxide filling the pressure vessel were changed from 40 ° C. and 20 MPa to those shown in Table 4. By performing the same operation as described above, a foam was obtained. Using a scanning calorimeter, the Tm and crystallinity of the obtained foam were measured. Also, the SE of the cross section of the obtained foam was
The average cell diameter D and cell number density N of the cells of the foam were determined from the M photograph by image processing. Table 5 shows the results.

[0040]

[Table 3] * 3. Since the carbon dioxide impregnation time is sufficient, it is considered that the sheet thickness does not affect the experimental results.

[0041]

[Table 4]

[0042]

[Table 5] * 4. In the solvent resistance test, ○ indicates that the trace of acetone dripping was small and inconspicuous, △ indicates that the trace was slightly large, and X indicates that the trace was large and clearly recognized.

Claims (9)

[Claims] 1. A method for producing a crystalline methacrylic resin, comprising a step of bringing a mixture of an isotactic methyl methacrylate polymer and a syndiotactic methyl methacrylate polymer into contact with a high-pressure gas. 2. A process for contacting a mixture of an isotactic methyl methacrylate-based polymer and a syndiotactic methyl methacrylate-based polymer with a high-pressure gas and then foaming the mixture. Production method. 3. The method for producing a crystalline methacrylic resin according to claim 1, wherein the high-pressure gas to be contacted contains 50% by volume or more of carbon dioxide. 4. The method for producing a crystalline methacrylic resin according to claim 1, wherein the pressure of the high-pressure gas to be contacted is 1 MPa or more. 5. The method for producing a crystalline methacrylic resin according to claim 1, wherein the temperature of the high-pressure gas to be contacted is 0 ° C. or higher. 6. The method for producing a crystalline methacrylic resin according to claim 1, wherein the high-pressure gas to be brought into contact is in a supercritical state. 7. The method for producing a crystalline methacrylic resin according to claim 1, wherein the mixture is at least one of a melt, a film, a plate, and various molded articles. Method. 8. A crystalline methacrylic resin foam having fine cells having an average cell diameter of about 10 μm or less and a cell number density of 10 ⁹ to 10 ¹⁵ cells / cm ³ . 9. The method according to claim 8, wherein a mixture of the isotactic methyl methacrylate polymer and the syndiotactic methyl methacrylate polymer is brought into contact with a high-pressure gas, and then the mixture is foamed. Crystalline methacrylic resin foam.