JPH0252653B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a foam made of a hard thermoplastic resin that has ultra-low density, high flexibility, high sound insulation performance, and high heat insulation performance, and a method for manufacturing the same. BACKGROUND ART Conventionally, rigid thermoplastic resin foams have been widely used as insulation materials for buildings such as general residences due to their characteristics such as low thermal conductivity, low water absorption, lightness, and workability. However, it is less flexible,
In addition, because the compression set is large, for example, in order to fill between the studs of a building, it is necessary to cut it in advance to match the stud spacing and fix it with special metal fittings. It cannot be absorbed, the workability is extremely poor, and an airtight insulation structure cannot be obtained. Furthermore, if an attempt is made to fill the material airtight, a great deal of force is required due to its hardness, resulting in the destruction of the bubbles. Furthermore, the foam also breaks, making it impossible to obtain an airtight heat-insulating structure. On the other hand, with the recent rise in the height of residential buildings and other buildings,
Especially in buildings such as apartment complexes, sound insulation between upper and lower floors is increasingly required. In order to solve this problem, a method called the floating floor construction method has recently been adopted, in which inorganic fiberboard is placed on a floor base and concrete mortar is poured on top of it through a waterproof layer. This utilizes the flexibility of inorganic fiberboard,
This can reduce solid-borne sound between the upper and lower floors and improve sound insulation. Even in this application, conventional rigid thermoplastic resin foams cannot provide sufficient sound insulation due to their rigidity. Recently, in order to solve these problems mentioned above,
For example, there are foams obtained by mechanically softening low-density polystyrene foam, but in this case, when the density becomes less than 20Kg/ ^m3 , the resin film that makes up the cells is removed due to the mechanical flexibility. This causes the so-called communication phenomenon. Furthermore, even if the relatively thick surface layer of the foam suppresses the destruction of the bubbles and maintains the apparent closed cell ratio as seen from the water absorption rate using a general measurement method, the inside of the foam The bubbles, which are made up of a thin film compared to the surface layer, are destroyed, resulting in a foam that is far inferior in terms of insulation performance and long-term insulation performance. Temperature mobility increases. Furthermore, when it is mechanically softened, the foam as a whole is softened, and when viewed microscopically, the softened portion of the foam's cells,
There are some parts that are not covered, and the physical properties thereof become non-uniform in the thickness direction or in the planar direction of the foam. On the other hand, foamable particles containing a foaming agent are foamed in several batches, and further foamed in a mold to form a plate-like object.
Alternatively, some molded bodies are used as heat insulating materials, cushioning materials, or sound insulating materials. However, in foams like this one, in which the particles themselves are made to have a low density (highly foamed) by increasing the number of times they are foamed, the particles themselves become large, making it difficult to airtightly fill them when molded.
The result is a foam with weak fusion between particles. In addition, in order to strengthen the fusion bond, it is necessary to perform compression filling and compression molding when making a molded product.
The foam made in this way has no wrinkles as defined in the present invention, and lacks flexibility and sound insulation, so there is a limit to how low the density can be made. The density of foam that can withstand practical use is only 17 kg/m ³ or more. However, in the market, there are products that have low density, are highly flexible, have heat insulation performance, maintain heat insulation performance for a long period of time, have low temperature transferability of insulation performance, have sound insulation properties, and have low compression set. Obtaining rigid thermoplastic foams has been a strong desire for many years. The present invention was achieved as a result of research in view of the current situation, and is a novel and inventive material with unprecedented properties that can be used as a heat insulating material, a sound insulating material, and a cushioning material. It is an invention. In other words, it is made by foaming a hard thermoplastic resin and has a density (D) of 3Kg/m ³ ≦D≦17Kg/m ³ . It has one end and a number of wrinkles extending toward the central part of the cell membrane, and the average cell diameter (A) is
A≦2.0mm, closed cell ratio (B) is B≧50%, and dynamic spring constant (k) is k≦40×10 ⁶ N/m ³ , and measured in three mutually orthogonal axes directions. The present invention provides a foam in which the minimum value (C) of the 60% compression set values is C≦25%. In addition, the present invention regarding the manufacturing method takes advantage of the extremely high permeability of water vapor to the resin, and combines the temperature mobility of the gas permeability to the resin with the softening temperature of the resin to produce an ultra-low density product. The inventors have now invented a method for obtaining a hard thermoplastic resin foam that is highly flexible, can maintain high heat insulation performance for a long period of time, has excellent sound insulation properties, and has low compression set. That is, a rigid thermoplastic resin foam is heated in hot water of 85°C or higher or in a steam atmosphere for 1 minute or more, and once expanded, it is shrunk to 70% of the final foam volume, and then Ambient temperature (T)
However, in a drying room at 40℃≦(T)<resin softening temperature 24
Aged for more than an hour and expanded to a density (D) of 3Kg/m ³
The cell structure has one end at the boundary where at least three bubbles are formed adjacent to each other, has many wrinkles extending toward the center of the cell membrane, and has an average cell diameter (A) of A≦ 2.0mm, closed cell ratio (B) is B≧50%,
and the dynamic spring constant (k) is k≦40×10 ⁶ N/
m ³ and 60% compression set measured in three axial directions perpendicular to each other, the minimum value (C) is C≦25%. It is something to do. Further, the contents of the present invention will be explained in detail with reference to the drawings. First, the foam density (D) of the hard thermoplastic resin, which is the first component of the present invention, is 3Kg/m ³ ≦D≦17Kg/m ³
The cell structure has one end at the boundary where at least three cells are adjacent to each other, and has a large number of wrinkles extending toward the center of the cell membrane. Conventional rigid thermoplastic resin foams generally have a high density, do not have the wrinkles referred to in the present invention, and have poor flexibility. Even if the density is low, since there are no wrinkles as defined in the present invention, once a load is absorbed, the cells in the foam burst, resulting in large permanent deformation and poor recovery properties. In addition, the conventional method requires a large compression force when compressing and filling between the studs of a building, making it difficult to perform compression and filling by hand, resulting in extremely poor workability. The body itself was likely to be destroyed, reducing not only the shock absorbing performance but also the heat insulating performance. In contrast, the product of the present invention can be compressed and filled with low compression force due to the above-mentioned density and the presence of wrinkles, and the cell structure of the foam can be destroyed because the stress is absorbed by the wrinkles. This allows filling work to be carried out without any problems. In addition, even if it is subjected to a load, it absorbs it through wrinkles and has low permanent deformation, so it has excellent recovery properties, can withstand repeated use, and has long-term cushioning and insulation performance. It can be maintained at a high level. In order to further clarify the presence and nature of wrinkles, Figure 1 shows an enlarged photograph of the cell structure of the product of the present invention and a comparative product of a mechanically wrinkled foam. . FIG. 1a shows a product of the present invention, in which the cell membrane itself is thin, has one end at the boundary where at least three bubbles are formed adjacent to each other, and has a large number of wrinkles extending toward the center of the cell membrane. I know there is. In contrast, the mechanically wrinkled one shown in Figure 1b is
Wrinkles are present in a band shape in the foam, with some parts having wrinkles and other parts not having wrinkles. This has a great influence on flexibility and compression set. From the above, it can be seen that the foam of the present invention requires the density and wrinkle structure described above. Even if the foam of the present invention has the above-mentioned density and the presence of wrinkles within the range of the present invention, the foam has an average cell diameter of
It is necessary that (A) satisfies (A)≦2.0 mm and that the closed cell ratio (B) satisfies (B)≧50%. The reason for this is that when the average bubble diameter (A) is 2.0 mm < (A), the convection of gas within the bubbles becomes large and the insulation performance is significantly reduced. In addition, when the closed cell ratio (B) is (B) < 50%, the water absorption rate of the foam increases, and the deterioration of the insulation performance due to water absorption increases, and from the viewpoint of maintaining insulation performance, it is difficult to use the insulation as a practical material. This is because it becomes something that is not suitable for use. The closed cell ratio is more preferably (B)≧70%. Furthermore, even if the density, the presence of wrinkles, and the foam structure are within the range of the present invention, the foam of the present invention has a dynamic spring constant (k) of k≦40×10 ⁶ N/m ³ and mutually orthogonal 3
It is necessary that the minimum value (C) of the 60% compression set measured in the axial direction is C≦25%. The reason for this is, for example, as mentioned above, in the floors of buildings such as apartment complexes, the floating floor construction method, in which the floors are constructed with cushioning materials, is effective against noise generated by floor impact noise between upper and lower floors. It is highly praised for its sound insulation properties. Figure 2 is a cross-sectional view of the main part of the floor showing an example of the floating floor construction method. determined by. That is, it is necessary to reduce the dynamic spring constant of the floating floor system in order to reduce the vibration generated by a large impact force on the floor with the sound insulating material (buffer material) 2 so that it does not directly propagate to the frame. Generally, the dynamic spring constant of this floating bed system is 30
It is said that it is better to have a value of ×10 ⁶ N/m ³ or less, and the smaller the value, the more effective it is. Here, the dynamic spring constant of this system is the dynamic spring constant of the sound insulation material 2,
It is determined by the weight (area density) of the floor plate 3 such as holding concrete. Generally speaking, when considering a practical floating floor, the areal density of the floor plate is in the range of 50 to 300 Kg/m ² in consideration of rigidity and economic efficiency. In this case, in order to make the dynamic spring constant of the floating floor system less than 30×10 ⁶ N/m ³ , the dynamic spring constant of the sound insulation material determined by the measurement method described later must be 40×10 ⁶ The value must be less than N/m ³ (at a thickness of 5 cm and an areal density of 250 Kg/m ² ). Regarding compression set, a foam with a compression set of 60% or more exceeding 25% will cause cell collapse or chipping of the foam due to stress when compressed and filled between studs, for example. The reason why the minimum value of the compression set in the three mutually orthogonal axes directions is indicated in the present invention is that the direction of flexibility required for the foam of the present invention may differ depending on its use. Generally, it is preferable to have flexibility almost equally in all directions. As mentioned above, the foam of the present invention is a novel foam that has not been seen before, and if necessary, it can be used as a composite with a plastic board, plastic film, wood board, inorganic material, cloth, etc. However, it is effective because it can provide excellent strength, heat insulation, and sound insulation properties. Next, regarding the manufacturing method of the present invention, firstly, the density is preferably 200Kg/m ³ or less, and more preferably the density is 100Kg/m 3 or less.
First, a rigid thermoplastic resin foam weighing less than Kg/ ^m3 is
It is necessary to heat for 1 minute or more in a hot water or steam atmosphere of 85°C or higher. The reason for this is that in a hot water or steam atmosphere of less than 85°C, the foam density hardly changes (decreases) even if heated for a long time. This will be further explained with reference to FIGS. 3 and 4. Figure 3 shows that an extruded polystyrene foam board with an initial density of 34 kg/m ³ and a thickness of 15 mm is placed in a steam chamber, the density is reduced while measuring the ambient temperature moment by moment, the density is reduced by the method of the present invention, and the final foam density is This is a graph plotting the measured values. According to this graph, when foaming is carried out in a steam atmosphere at 83℃,
There is almost no change in the density of the foam even when heated for a long time, but when the temperature exceeds 85°C, the density of the foam decreases as the heating time increases. Ambient temperature is 85
It is sufficient to heat the foam at a temperature of 1 minute or more at a temperature of 0.degree. C. or higher, but the heating time varies depending on the material, thickness, etc. of the foam before heating, and from an industrial perspective, it is generally preferable to select a heating time of 60 minutes or less. FIG. 4 is a graph of the final foam density when the same foam as above was heated using air at 90°C, hot water at 90°C, and steam at 90°C as heating sources. According to this graph, it can be seen that the density of the foam decreases in hot water or steam, but hardly occurs in air. From the above, it can be seen that the method of the present invention requires heating in a hot water or steam atmosphere of 85° C. or higher for at least 1 minute, preferably within 60 minutes. Next, after performing the above heat treatment, it is necessary to shrink the final foam to 70% or less of its volume (in this case, the volume immediately after heating and foaming should be used for comparison, but When exposed to the atmosphere, it shrinks, making it extremely difficult to measure its dimensions, and at the same time making the measured value itself inaccurate, so we purposely used the volume of the final foam for comparison.) The reason for this is that after the above heating and foaming treatment,
If the volume once shrunk exceeds 70% of the final foam volume, i.e. if the amount of shrinkage is kept low, wrinkles may not form in the cell membrane, or even if wrinkles do not form uniformly throughout the foam. First, it is impossible to obtain a foam having the desired physical properties. However, at present, as mentioned above, the method of achieving low density is to heat and foam expandable particles containing a foaming agent in several batches, and then further heat and foam them in a mold so that there is almost no dimensional change after foaming. Since the foam is foamed to form a plate-like body or a molded body, the foamed body has almost no wrinkles as defined in the present invention. In order to further clarify the above contents, the fifth
The figure shows a photo of the cell structure of the foam obtained by the method of the present invention, and a photo of the cell structure of the foam obtained by the method of the present invention.
A photograph (b) of the cell structure of the foam obtained by heating and foaming in two steps (b) and (b) is shown. The product of the present invention shown in a is:
There are a number of wrinkles that have one end at the boundary where at least three bubbles occur adjacent to each other and extend toward the center of the bubble membrane, but the comparative product shown in b does not have any wrinkles as defined in the present invention. I understand. Furthermore, in the method of the present invention, the once-shrinked foam is dried in a drying room where the ambient temperature (T) is 40°C≦T≦the softening temperature of the resin, preferably at a humidity of 30% or less, more preferably at a humidity of 10% or less. It needs to be aged indoors for at least 24 hours. The reason for this is that the atmospheric temperature (T) for recovery is 40
This is because if the temperature is less than 0.degree. C., as shown in FIG. 6, it takes a long time to recover, which is disadvantageous industrially. This is clear from the gas permeation curve of air against polystyrene shown as an example in FIG. In other words, this means that the amount of air permeated through polystyrene increases at 40°C, and the recovery speed becomes faster. On the other hand, if heated to a temperature higher than the softening temperature of the resin, the resin itself will melt at the same time as recovery, making it impossible to form a foam. In addition, if the humidity is high, it is difficult to replace the moisture in the shrunken foam with air, and after the recovery operation is completed,
When removed from the heated atmosphere, it contracts again and
It becomes difficult to obtain dimensionally stable foams. Furthermore, the aging time is determined by the temperature and the density of the foam to be obtained, but even when obtaining a foam of 17 kg/m ³ , the method of the present invention can be used at temperatures close to the softening temperature of the resin. However, at least 24 hours are required. The recovery atmosphere described above is a dry air atmosphere as an example, but inorganic gases such as carbon dioxide, helium, and hydrogen, or organic gases,
Depending on the application, it can be used even when mixed with air. In addition, in the method of the present invention, during heating and foaming,
It is more preferable to provide an auxiliary plate in order to maintain the product shape of the foam well during shrinkage and aging. In addition, when giving directionality to the physical properties of the foam,
It is also possible to suppress foaming in one or both of the width, length, and thickness directions by placing the foam in the mold frame, and to allow free foaming only in the remaining two or one direction. The hard thermoplastic resin as used in the present invention refers to styrene, methylstyrene, ethylstyrene, chlorostyrene, or alkenyl aromatic compounds such as those mentioned above and other easily polymerizable olefin compounds.
For example, copolymers with maleic anhydride, acrylic acid, methacrylic acid, etc., so-called styrenic polymers such as rubber reinforcing polymers, or acrylic copolymers such as acrylonitrile, methyl methacrylate, and acrylonitrile-butadiene-styrene copolymers. , polycarbonate, polyphenylene oxide, rigid vinyl chloride polymer, or a mixture of the above polymers. What is rigid thermoplastic foam?
It refers to a foam obtained by extrusion foaming, in-mold foaming, or free foaming of the above polymer or a mixture thereof using a chemical foaming agent, a physical foaming agent, or a mixture thereof.
Particularly preferred is a so-called extruded foam obtained by melt-kneading the above-mentioned styrenic polymer with a physical blowing agent, a chemical blowing agent, or a mixture thereof in an extruder, and extruding it through a T-die or an annular die. It also includes foamed materials formed by foaming and fusing expandable particles in a mold to form a plate-like object or other molded object, but in this case, the fusion between the particles becomes somewhat poor, and the foaming is forced into a foamed or sheet-like form. The heat insulation properties are somewhat inferior to those obtained using foamed materials. Further, the shape of the hard thermoplastic resin foam used in the method of the present invention is not particularly limited, but plate-like, prismatic, and sheet-like forms are effective, and the thickness is 50 mm.
The following are particularly effective in maintaining the dimensional accuracy of the final foam. In addition, the foam may contain general nucleating agents, lubricants, colorants, ultraviolet absorbers, etc. as necessary.
It may also contain an antistatic agent. The methods for measuring the foam density, average cell diameter, closed cell ratio, dynamic spring constant, 60% compressive strain, and resin softening temperature used in the present invention are based on the following methods. Γ Foam density: JIS-A-9511 Γ Average cell diameter: Measured in the horizontal and vertical directions of the foam in the thickness direction of the foam and on the same plane orthogonal to the thickness direction according to JIS-K-6402 Then, the values in each direction are summed and averaged. Γ Closed cell ratio: Measured according to the measurement method of ASTM-D-2856 (including the value of the open cell layer on the sample surface layer). Cut each surface of the sample into 1/20th of the thickness in the thickness direction of that surface,
Measure again. This operation is repeated n times and expressed as the average value of the n times. The more times the better,
Since there is a limit depending on the size of the original sample, at least n≧3. ΓDynamic spring constant: Obtained from the test method below. (i) Testing Apparatus In this testing apparatus, a synthetic resin foam test piece 9 is placed on a surface plate 10 as shown in FIG. 8, and a load plate 11 is placed on top of it. A vibration pickup 12 connected to a waveform recording device 14 is arranged on the load plate 11. Further, an amplifier 13 is installed between the waveform recording device 14 and the vibration pickup 12. (a) Dimensions of synthetic resin foam test product 9: 500mm x 500mm x 50mm (thickness) (b) Surface plate 10: Flatness of 1mm or less, inclination within ±1° with respect to the horizontal plane, and sufficient effective mass (c) Load Plate 11 Flatness 0.2mm or less, size 300mm ± 3mm square, mass 22.5Kg (= 250Kg/m ² ) error ±
1% or less and does not cause harmful bending vibrations (d) Vibration Pickup 12 Use something as light as possible so as not to affect damped vibration. (e) Vibration waveform recording device A device capable of observing the waveform of natural vibration (ii) Measurement method A softball baseball ball is caused to fall freely vertically from a height of approximately 0.8 m to the center of the load plate 11, and is then vibrated. Observe the waveform. (iii) Calculation method of dynamic spring constant per unit area No. 9
If the period T is 2 or more (T ₁ , T ₂
...) is read, and the value obtained from the average value using the following formula is the dynamic spring constant k per unit area. k = (2π1/Tn) ² × m (N/m ³ ) m = Load mass per unit area (250Kg/m ³ ) Tn = Average value of natural period (seconds) Γ60% compression set: JIS-K- According to 6767 Softening temperature of Γ resin: ASTM-D-1525 Each evaluation item used in the present invention is based on the following evaluation method and evaluation scale. Cushioning property -1 Cushioning property (compression set) Γ evaluation method: According to JIS-K-6767 (compression creep test method), two directions perpendicular to each other on the same plane perpendicular to the thickness direction and the thickness direction of the foam , that is, 60 in three directions, length direction and width direction.
The % compression set was measured and evaluated based on its minimum value (C). Γ rating scale:

[Table] -2 Cushioning property (dynamic buffering property) Γ evaluation method: According to the measurement method of JIS-Z-0235 (evaluated with static stress of 0.02Kg/ ^cm2 or more), thickness 50mm (thickness less than 50mm is overlapping) The average value of the maximum deceleration (G) during 2 to 5 falls was determined and evaluated. In addition, when the static stress was 0.02 Kg/cm ² or less, it was marked (×). Γ rating scale:

[Table] -3 Cushioning property (filling workability (low compressive stress property)) Γ evaluation method: According to JIS-K-6767 (compression creep test method), in the thickness direction of the foam and on the same plane perpendicular to the thickness direction The stress was measured when 10% compressive strain was applied in two orthogonal directions, that is, the length direction and the width direction, and the stress was evaluated based on the minimum value (P) in the three directions. Γ rating scale:

[Table] -4 Amount of bending deflection Γ Evaluation method: Tested according to the method of JIS-A-9511, and evaluated based on the magnitude of the maximum deflection (y) determined by the following formula. y=P/E・l ³ /4bh ³ P: Maximum load (Kg) l: Span distance (cm) b: Width of test piece (cm) h: Thickness of test piece (cm) E: Flexural modulus ( Kg/ ^cm2 ) y: Maximum deflection (cm) Γ evaluation scale:

[Table] Thermal insulation-1 Thermal conductivity Γ Evaluation method: According to ASTM-C-518,
Kcal/m・hr℃ and 0
Evaluation was made using the value of °C (λ). Γ rating scale:

[Table] -2 Temperature gradient of thermal conductivity Γ evaluation method: ASTM-C- as in -1
518, two or more points at different temperatures (this method is 15℃, 35℃, and 55℃)
The temperature gradient (x) of thermal conductivity was determined and evaluated. Γ rating scale:

[Table] Sound insulation Γ Evaluation method: Measured according to JIS-A-1418 floor impact sound level measurement method, JIS-A
-1419 was evaluated based on the nomenclature of the sound insulation grade regarding the floor impact sound level.
The test specimen was a 50mm thick flat plate of foam placed on top of an RC building floor frame, and a 100μ thick foam was placed on top of the 50mm thick flat foam.
After applying a polyethylene film, concrete mortar was applied to a thickness of 50 mm on top of the concrete mortar, and after curing for a week, the floor was finished with a 3 mm thick needle punch carpet to reduce the floor impact noise level. I took measurements. Γ rating scale:

[Table] Example/Comparative Example 1 Thickness 25mm, width 400mm, length 700mm, density 22-28
Kg/m ³ of extruded polystyrene foam was placed in a heating furnace, and the heating medium, heating temperature,
Table 1 summarizes the structure and evaluation results of the foams obtained by changing the heating time and keeping the aging conditions constant at 75°C in air for 120 hours.

【table】

[Table] Example/Comparative Example 2 Thickness 15mm, width 300mm, length 500mm, density 25-110
Kg/m ³ of extruded foam was placed in a heating pot in the same manner as in Example and Comparative Example 1, the heating medium was limited to steam and hot water, the heating temperature was 90℃ and 100℃, and the heating time was also 15 minutes and 30 minutes.
As a result of looking at the difference in the ratio of the foam volume at the time of shrinkage to the foam volume at the end of aging with the aging conditions kept constant, the obtained foam structure and evaluation results are summarized in Table 2. . The resins used were polystyrene, polymethyl methacrylate, and styrene methacrylic acid copolymer as shown in Table 2.

【table】

[Table] Example/Comparative Example 3 An extruded polystyrene foam with a density of 22 to 28 kg/m ³ having the same sample size as Example/Comparative Example 1 was heated under foaming conditions (heating medium, heating temperature, heating time). Table 3 summarizes the structures and evaluation results of the foams obtained by changing the aging conditions (temperature, time). Note that two types of polystyrene with different softening temperatures were used.

【table】

[Table] Example/Comparative Example 4 The foam of the present invention has all the evaluations of the present invention, and in order to clarify how this foam is positioned compared to current commercially available products. The following foamed boards were evaluated using the various evaluation methods described in the text. The evaluation results are summarized in Table 4. ΓFoam of the present invention (representative) No. 1, 15, 22 ΓCommercial product

【table】

【table】

[Table] By having the above-mentioned structure, the present invention becomes a foam that is highly flexible, has excellent heat insulation properties, has sound insulation properties, and has low compression set, and can be used as insulation for floors, walls, roofs, etc. of buildings. This invention is useful for industry as it has many advantages as a sound insulating material or a cushioning material.

[Brief explanation of the drawing]

Figure 1 a is an enlarged photograph showing the cell structure of the product of the present invention, b is an enlarged photograph showing the cell structure of the comparative product, and Fig. 2
The figure shows an example of the floating floor construction method, Figure 3 is a graph showing an example of the density reduction of foam due to heating and foaming, Figure 4 is a graph showing an example of the density reduction due to different types of heating medium, and Figure 5 a is an enlarged photograph showing the cell structure of the product of the present invention, b is an enlarged photo showing the cell structure of the comparative product, Fig. 6 is a graph showing the recovery state of the foam at different aging temperatures, and Fig. 7 is water vapor against the polystyrene membrane. Graph showing an example of temperature mobility of permeation, No. 8
9 and 9 are explanatory diagrams regarding the dynamic spring constant measurement method.

Claims (1)

[Claims] 1. Made by foaming a hard thermoplastic resin, the density (D) is 3Kg/m ³ ≦D≦17Kg/m ³ , and in the cell structure, at least three cells are adjacent to each other. The cell membrane has many wrinkles that have one end at the boundary and extend toward the center of the cell membrane, and the average cell diameter (A) is A≦
2.0mm, the closed cell ratio (B) is B≧50%, and the dynamic spring constant (k) is k≦40×10 ⁶ N/m ³ , and measured in three mutually orthogonal axes directions. A foam whose minimum value (C) of the 60% compression set values is C≦25%. 2. Heat the hard thermoplastic resin foam in hot water of 85°C or higher or in a steam atmosphere for 1 minute or more, expand it, and then shrink it to 70% or less of the final foam volume, and then heat it in the atmosphere. Temperature (T)
However, in a drying room at 40℃≦(T)<resin softening temperature 24
Aged for more than an hour and expanded until the density (D) is 3Kg/m ³ ≦
D≦17Kg/ ^m3 , and at least 3 in the cell structure
It has one end at the boundary where two bubbles are formed adjacent to each other, has many wrinkles extending toward the center of the bubble membrane, has an average cell diameter (A) of A≦2.0mm, and has a closed cell ratio (B). is B≧50%, and the dynamic spring constant (k) is k≦40×10 ⁶ N/m ³ , and the value of 60% compression set measured in three mutually orthogonal directions. Minimum value (C)
A method for producing a foam, characterized in that a foam is obtained in which C≦25%.