Patent Publication Number: US-4585605-A

Title: Rigid thermoplastic resin foam and process for preparation thereof

Description:
This is a division of application Ser. No. 717,268, filed Mar. 19, 1985, now U.S. Pat. No. 4,552,904. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a foam of a rigid thermoplastic resin, which has an ultra-low density, a high flexibility, a high sound-insulating property and a high heat-insulating property, and also to a process for the preparation of this foam. 
     BACKGROUND ART 
     Heretofore, rigid thermoplastic resin foams have been widely used as heat-insulating materials for ordinary houses and other buildings because of such excellent properties as a low heat conductivity, a low water-absorbing property, a light weight and a good processability. However, since these foams are poor in the flexibility and have a large compression set, for example, when these foams are filled between pillars in buildings, it is necessary to cut the foams into a size corresponding to the distance between the pillars and fix the cut foams by special metal fittings. Practically, deviations of the distance between the pillars are not compensated, the operation efficiency is very low, and an air-tight, heat-insulating structure cannot be obtained. Furthermore, since they are rigid, a large force is necessary for filling the foams air-tightly, and application of a large force results in breakage of cells. Moreover, it happens that even the foams per se are broken, and an air-tight, heat-insulating structure cannot be obtained. 
     Recently, the heights of houses and buildings are increased, and especially in such buildings as multi-storied apartment houses, a high sound-insulating property is required between two adjacent stories. In order to solve this problem, there has recently been adopted a floating floor construction method in which an inorganic fiber board is arranged on a floor base and a concrete mortar is deposited on the inorganic fiber board through a waterproof layer. According to this method, the flexibility of the inorganic fiber board is utilized and solid-propagative sounds between upper and lower stories are reduced, and the sound-insulating property can be enhanced. When the conventional rigid thermoplastic resin foams are used in this method, since they are rigid, no sufficient sound-insulating property can be obtained. 
     As means for eliminating the foregoing disadvantages, there has recently been proposed a foam obtained by mechanically softening a polystyrene foam having a reduced density. In this foam, however, if the density is lower than 20 Kg/m 3 , the resin wall defining cells is broken by the mechanical softening treatment, and a so-called communicating phenomenon takes place. Furthermore, even if breakage of cells in a relatively thick surface layer of the foam is controlled and an apparent closed cell ratio judged from the water absorption determined by an ordinary measuring method is maintained, cells which are present in the interior of the foam and composed of a wall thinner than the wall of the surface layer are broken, and the foam is inferior in the heat-insulating property and the maintenance of the heat-insulating property for a long time and the temperature dependency of the heat-insulating property increases. Furthermore, if the mechanical softening treatment is carried out, the foam as a whole is softened, and microscopically, the cells of the foam include softened portions and unsoftened portions and the physical properties of the foam are not uniform in the thickness direction or plane direction of the foam. 
     Plates or molded articles obtained by blowing foamable particles containing a foaming agent several times separately and finally effecting blowing in a mold are used as heat-insulating, shock-absorbing or sound-insulating materials. However, in a foam having a low density (a high blowing ratio) which is obtained by increasing the frequency of the operation of blowing the particles, since the size of the particles per se is increased, air-tight filling is impossible when a molded article is formed, and the fusion bonding among the particles is weak in the obtained foam. In order to obtain a strong fusion bonding, it is necessary to perform compressive filling and compressive molding when a molded article is formed, and the obtained foam is free of wrinkles referred to in the present invention and is poor in the flexibility and sound-insulating property. Accordingly, in the above-mentioned method, reduction of the density is restricted and the density of a foam that can be put into practical use is 17 Kg/m 3  or higher. 
     However, it has long been desired in the market to develop a rigid thermoplastic resin foam having a low density, a high flexibility and a good heat-insulating property, where the heat-insulating property is maintained for a long time, the temperature dependency of the heat-insulating property is small, the sound-insulating property is good and the compression set is reduced. 
     DESCRIPTION OF THE INVENTION 
     The present invention has been completed as the results of researches made in view of the above circumstances. The present invention provides a novel and progressive foam having excellent properties not possessed by the conventional heat-insulating, shock-absorbing or sound-insulating materials. 
     More specifically, the present invention provides a foam obtained by foaming a rigid thermoplastic resin, said foam having a density (D) of 3 Kg/m 3  ≦D≦17 Kg/m 3 , a multi-cellular structure including many wrinkles having one end in the boundary portion where at least 3 cells are adjacent to one another and extending toward the central portion of the cell wall, an average cell diameter (A) of A≦2.0 mm, a closed cell ratio (B) of B≧50%, a kinetic spring constant (K) of K≦40×10 6  N/m 3 , and a minimum value (C), among 60% compression set values determined with respect to three directions rectangular to one another, of C≦25%. 
     Furthermore, according to the preparation process of the present invention, a high permeability of water vapor through a resin is utilized and the temperature dependency of the permeability of gases through the resin is combined with the softening point of the resin, whereby is obtained a rigid thermoplastic resin foam having an ultra-low density and a high flexibility, in which a heat-insulating property is maintained for a long time, the sound-insulating property is excellent and the compression set is reduced. 
     More specifically, in accordance with another aspect of the present invention, there is provided a process for the preparation of a rigid thermoplastic resin foam, which comprises heating a foam of a rigid themoplastic resin for at least 1 minute in hot water maintained at a temperature higher than 85° C. or water vapor atmosphere maintained at a temperature higher than 85° C. to expand the foam, then contracting the foam so that the volume is reduced to less than 70% of the volume of the final foam, and aging and expanding the foam in a drying chamber at an ambient temperature (T) of 40° C.≦T&lt; the softening point of the resin for at least 24 hours, whereby is obtained a foam having a density (D) of 3 Kg/m 3  ≦D≦17 Kg/m 3 , a multi-cellular structure including many wrinkles having one end in the boundary portion where at least 3 cells are adjacent to one another and extending toward the central portion of the cell wall, an average cell diameter (A) of A≦2.0 mm, a closed cell ratio (B) of B≧50%, a kinetic spring constant (K) of K≦ 40×10 6  N/m 3 , and a minimum value (C), among 60% compression set values determined with respect to three directions rectangular to one another, of C≦25%. 
     The present invention will now be described in detail with reference to the accompanying drawings. 
     The first structural requirement of the present invention is that the density (D) of the rigid thermoplastic resin foam is in the range of 3 Kg/m 3  ≦D≦17 Kg/m 3  and the rigid thermoplastic resin foam has a multi-cellular structure including many wrinkles having one end in the boundary portion where at least three cells are adjacent to one another and extending toward the central portion of the cell wall. 
     A conventional ridid thermoplastic resin foam ordinarily has a high density and is free of wrinkles referred to in the present invention, and it is inferior in the flexibility. Even if the density is low in the conventional foam, since there are not wrinkles referred to in the present invention, when a load is once absorbed in the foam, cells in the foam are broken, with the result that a large compression set is brought about and the recovery property is insufficient. Furthermore, when the conventional rigid thermoplastic resin foam is filled under compression between pillars of a building, a large compressive force is necessary, and the compression filling by the manual operation is difficult and the operation efficiency is very low. Moreover, the foam per se is broken by the compressive force applied for the compression filling, and not only the shock-absorbing property, but also the heat-insulating property is readily degraded. 
     In contrast, by dint of the above-mentioned ultra-low density and the presence of the wrinkles, the foam of the present invention can be filled with a small compressive force and since the stress generated at the compression filling is absorbed by the wrinkles, the filling operation can be accomplished without breakage of the cellular structure of the foam. Moreover, even if a load is imposed on the foam, the load is absorbed by the wrinkles and the compression set is small, and therefore, the foam has an excellent recovery property and can resist repeated use. Furthermore, both the shock-absorbing property and the heat-insulating property can be maintained at high levels for a long time. 
     For further clarification of the significance of the presence of the wrinkles and the manner of the arrangement of the wrinkles, FIG. 1 includes an enlarged photo showing the cellular structure of the foam of the present invention and an enlarged photo showing the cellular structure of a comparative foam which is mechanically wrinkled. 
     In the foam of the present invention shown in FIG. 1-(a), it is seen that the cell wall is thin, and there are present many wrinkles having one end in the boundary portion where at least three cells are adjacent to one another and extending toward the central portion of the cell wall. In contrast, in the mechanically wrinkled foam shown in FIG. 1-(b), there are present wrinkles extending in the ribbon-like form and there are observed a wrinkle-present portion and a wrinkle-free portion. This feature has significant influences on the flexibility and compression set. In view of the foregoing, it is understood that the above-mentioned density and wrinkle structure are necessary in the foam of the present invention. 
     Even if the density is in the range specified in the present invention and the wrinkle structure is as defined in the present invention, an intended foam cannot be obtained unless the requirements of the average cell diameter (A) of A≦2.0 mm and the closed cell ratio (B) of B≧50% are satisfied. The reasons will be described below. 
     If the average cell diameter (A) is in the range of 2.0 mm&lt;A, the convection of gas in the cells becomes large and the heat-insulating property is drastically reduced. If the closed cell ratio (B) is in the range of B&lt;50%, the water absorption of the foam increases and the degradation of the heat-insulating property due to the absorption of water is increased, and since the maintenance of the heat-insulating property is insufficient, the foam cannot be practically used as a heat-insulating material. It is preferred that the closed cell ratio be in the range of B≧70%. 
     Moreover, even if the above-mentioned density, wrinkle and cellular structure requirements are within the scope of this invention, the foam of the present invention necessitates that the kinetic spring constant (K) should be in the range of K≦40×10 6  N/m 3  and the minimum value (C), among 60% compression set values determined with respect to three directions rectangular to one another, should be in the range of C≦25%. The reasons will be described below. 
     As pointed out hereinbefore, in such a building as a multi-storied apartment house, there is adopted a floating floor construction method in which a floor is boarded through a shock-absorbing material so as to reduce noises generated by shocks given to the floor between upper and lower stories, and the sound-insulating effect attained by this method is highly evaluated. FIG. 2 is a sectional view of a main part of a floor, which illustrates an example of the floating floor construction method. In this case, the sound-insulating property of the floating floor is mainly determined by the natural frequency (kinetic spring constant) of the floating floor system and the thickness of a floor body 1. More specifically, in order to damp vibrations generated by a large impact force on the floor by a sound-insulating material (shock-absorbing material) 2 for preventing the vibrations from being directly propagated to the floor body, it is necessary that the kinetic spring constant of the floating floor system should be reduced. It is considered that it is ordinarily preferred that the kinetic spring constant of the floating floor system be smaller than 30×10 6  N/m 3 , and the smaller is the value of the kinetic spring constant, the higher is the effect. The kinetic spring constant of the floating floor system is determined by the kinetic spring constant of the sound-insulating material 2 and the weight (plane density) of a floor board 3 of pressing concrete or the like. When a practical floating floor is taken into consideration, the plane density of the floor board is controlled in the range of 50 to 300 Kg/m 2  in view of the rigidity and from the economical viewpoint. In this case, in order to maintain the kinetic spring constant of the floating floor system at a level lower than 30×10 6  N/m 3 , it is indispensable that the kinetic spring constant of the sound-insulating material, determined according to the measuring method described hereinafter, should be 40×10 6  N/m 3  or less (when the thickness is 5 cm and the plane density is 250 Kg/m 2 ). A foam having a 60% compression set exceeding 25% is defective in that by a stress generated, for example, in the step of filling the foam under compression between pillars, cells are broken or the foam is chipped. The reason why the minimum value among compression set values determined with respect to three directions rectangular to one another is specified in the present invention is that in the foam of the present invention, the flexibility-requiring direction may differ according to the intended use. Ordinarily, it is preferred that the flexibility be substantially equal in all the directions. 
     The above-mentioned foam of the present invention is a novel foam which heretofore has not been provided at all. If the foam of the present invention is laminated with a plastic board, a plastic film, a wood board, an inorganic material or a cloth according to need, a product excellent in the strength, heat-insulating property and sound-insulating property can be obtained and high effects can be attained by this product. 
     As for the preparation process according to the present invention, at first, a rigid thermoplastic resin foam preferably having a density lower than 200 Kg/m 3 , more preferably lower than 100 Kg/m 3 , is heated for at least 1 minute in hot water maintained at a temperature higher than 85° C. or in a water vapor atmosphere maintained at a temperature higher than 85° C. The hot water of more than 85° C. naturally includes a system such as a hot water of 100° C. in which steam and liquid water coexist. 
     The above-mentioned heating condition is critical because if the temperature of hot water or the water vapor atmosphere is lower than 85° C., the density of the foam is hardly changed (reduced) even by conducting the heating for a long time. This condition will now be described with reference to FIGS. 3 and 4. 
     An extrusion-foamed polystyrene plate having 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, the foam is recovered according to the process of the present invention, and the density of the final foam is measured. The obtained results are plotted on a graph in FIG. 3. This graph shows that when blowing is effected in a steam atmosphere maintained at 83° C., even if the heating is effected for a long time, the density of the foam is hardly changed, but when the temperature is higher than 85° C., the density of the foam is reduced with the lapse of heating time. It is ordinarily sufficient if the heating is conducted at a temperature higher than 85° C. for at least 1 minute. However, the heating time is changed according to the material of the foam and the thickness of the foam before the heating, but from the industrial viewpoint, it is ordinarily preferred that the heating time is up to 60 minutes. FIG. 4 is a graph showing the densities of final foams obtained when the same foam as described above is heated by using air maintained at 90° C., hot water maintained at 90° C. and water vapor maintained at 90° C. as the heat source. This graph shows that reduction of the density of the foam is caused in hot water or water vapor but the density is hardly reduced in air. In view of the foregoing, in the process of the present invention, it is indispensable that the heating should be conducted in hot water or water vapor maintained at a temperature higher than 85° C. for at least 1 minute, preferably up to 60 minutes. 
     After the above-mentioned heat treatment, the foam should once be contracted so that the volume of the foam is reduced to less than 70% of the volume of the final foam (Although the volume just after the heating and blowing treatment should be used for the comparison, the volume of the final foam is used for the comparison in the present invention because if the foam is placed in air just after the heating, contraction takes place and the measurement of the size is very difficult, and the measured value per se is inaccurate). The reason for this contraction will now be described. 
     If the volume of the foam which is contracted after the heating and blowing treatment exceeds 70% of the volume of the final foam, that is, if the degree of the contraction is maintained at a low level, wrinkles are not formed on the cell wall, or they are not uniform throughout the foam even if wrinkles are formed, and a foam having the intended properties cannot be obtained. According to the above-mentioned density-reducing method adopted at present, foamable particles containing a foaming agent are heated and blown several times separately and the foam is further blown in a mold so that the size of the foam after the blowing is not substantially changed, whereby a plate or molded article is obtained. Accordingly, the foam obtained according to this conventional method does not substantially contain wrinkles referred to in the present invention. This will become more apparent from FIG. 5 including a photograph (a) of the cellular structure of the foam obtained according to the process of the present invention and a photograph (b) of the cellular structure of the foam obtained by heating and blowing the particles several times (three times) separately so that contraction does not take place. In the product of the present invention shown in (a), there are present many wrinkles having one end in the boundary portion where at least three cells are adjacent to one another and extending toward the central portion of the cell wall, but the comparative product shown in (b) does not contain wrinkles referred to in the present invention. 
     In the process of the present invention, it is indispensable that the foam which has thus been once contracted should be aged in a drying chamber maintained at an ambient temperature (T) of 40° C.≦T&lt; the softening point of the resin, preferably at the relative humidity lower than 30%, more preferably lower than 10%, for at least 24 hours. The reason for this condition will now be described. 
     If the temperature (T) of the recovering atmosphere is lower than 40° C., as shown in FIG. 6, a long time is necessary for the recovery and the process becomes industrially disadvantageous. This will also be apparent from FIG. 7 showing a curve of the gas permeation of air through polystyrene. Namely, if the temperature is 40° C. or higher, the amount of air permeating through polystyrene is increased, and the recovery speed of the foam is increased. If the foam is heated at a temperature higher than the softening point of the resin, the resin melts simultaneously with the recovery, and no foam can be obtained. If the relative humidity is high, substitution of water in the contracted foam with air is hardly advanced, and if the foam is taken out from the heating atmosphere after the recovery operation, contraction takes place again and it is difficult to obtain a foam excellent in the dimension stability. The aging time is determined according to the aging temperature and the intended density of the final foam. In the process of the present invention, the aging time should be at least 24 hours even in the case where a foam having a density of 17 Kg/m 3  is obtained and the aging temperature is close to the softening point of the resin. 
     In the foregoing illustration, air is taken as an instance of the drying recovery atmosphere. In the present invention, instead of air, there may be used an inorganic gas such as carbon dioxide gas, helium or hydrogen, or a mixture of an organic gas with air according to the intended use. In the process of the present invention, in order to impart a good shape to the product, it is preferred to dispose an auxiliary plate in the heating and blowing step, the contracting step and the aging step. 
     In the case where a directional property is given to the physical properties of the foam, there may be adopted a method in which expansion in one or two of the lateral, longitudinal and thickness directions is controlled in a molding frame and expansion is freely allowed in the remaining one or two directions. 
     As the rigid thermoplastic resin referred to in the present invention, there can be mentioned styrene type polymers such as homopolymers of styrene, methylstyrene, ethylstyrene and chlorostyrene, copolymers of these alkenyl aromatic compounds with easily copolymerizable olefin compounds such as maleic anhydride, acrylic acid and methacrylic acid, rubber-reinforced polymers, acrylic polymers such as polyacrylonitrile, polymethyl methacrylate and an acrylonitrile-butadiene-styrene copolymer, polycarbonates, polyphenylene oxides, rigid vinyl chloride polymers, and mixtures of two or more of the foregoing polymers. The rigid thermoplastic resin foam means a foam obtained by extrusion-blowing, blowing in a mold or freely blowing the polymer or polymer mixture as described above by a chemical foaming agent, a physical foaming agent or a mixture thereof. Especially preferred is a so-called extruded foam obtained by melt-kneading the aforesaid styrene type polymer with a physical foaming agent, a chemical foaming agent or a mixture thereof in an extruder and extruding the melt through a T-die or an annular die. A foam obtained by blowing and fusion bonding foamable particles in a mold to form a plate or other molded articles is included in the scope of the foam in the present invention, but fusion bonding of the particles is insufficient more or less and the heat-insulating property of this foam is inferior to that of a foam obtained by extrusion blowing or blowing in the form of a sheet. The shape of the rigid thermoplastic resin foam used in the process of the present invention is not particularly restricted, but a foam having a plate-like shape, a square pillar-like shape or a sheet-like shape is advantageously used. In order to maintain the precise dimension of the final foam, it is especially preferred that the thickness of the starting foam be smaller than 50 mm. Furthermore, the foam may incorporate therein ordinary additives such as a nucleating agent, a lubricant, a colorant, an ultraviolet absorber, and an antistatic agent when necessary. 
     In the present invention, the density of the foam, the average cell diameter, the closed cell ratio, the kinetic spring constant, the 60% compression set, and the softening point of the resin are determined according to methods described below. 
     Density of Foam: JIS A-9511. 
     Average Cell Diameter: According to the method of JIS K-6402, the cell diameter is measured with respect to each of the thickness direction of the foam and the lateral and longitudinal directions rectangular to the thickness direction on the same plane, and the mean value is calculated. 
     Closed-Cell Ratio: The closed-cell ratio is determined according to the measuring method of ASTM D-2856 (the value of open cells in the surface layer of the sample is included). Each surface of the sample is cut off in a thickness corresponding to 1/20 of the original thickness in a thickness direction to said surface, and the closed-cell ratio is determined again. This operation is repeated n times, and the mean value is calculated. Although the repetition frequency is preferred to be as large as possible, it is limited by the size of the original sample. Therefore, the repetition frequency n should be at least 3. 
     Kinetic Spring Constant: The kinetic spring constant is determined according to the following test method. 
     (i) Test Apparatus: 
     In this test apparatus, as illustrated in FIG. 8, a synthetic resin foam test piece 9 is placed on a platen 10, and a loading plate 11 is placed thereon. A frequency pick-up 12 connected to a wave form recording device 14 is arranged on the loading plate 11. An amplifier 13 is disclosed between the wave form recording device 14 and the frequency pick-up 12. 
     (a) Size of Synthetic Resin Foam Test Piece 9: 500 mm×500 mm×50 mm (thickness) 
     (b) Platen 10: The levelness is less than 1 mm, the inclination angle to the horizontal plane is within ±1°, and the platen has a sufficient effective mass. 
     (c) Loading Plate 11: The levelness is less than 0.2 mm, the plate has a square shape having a side of 300 mm±3 mm, the mass is 22.5 Kg (=250 Kg/m 2 ) with an error being within ±1%, and a harmful bending vibration is not caused. 
     (d) Frequency Pick-up 12: A pick-up having a weight as light as possible is used so that no influence is imposed on the damped oscillation. 
     (e) Frequency Wave Form Recording Device: A device enabling the observation of the wave form of the natural frequency is used. 
     (ii) Measuring Method: 
     A rubber baseball is freely dropped onto the center of the loading plate 11 in the vertical direction from a height of about 0.8 m to cause vibration, and the wave form is observed. 
     (iii) Method for Calculation of Kinetic Spring Constant per Unit Area: 
     At least 2 periods T (T 1 , T 2 , . . . ) are read between adjacent peaks of a damped wave form 15 of the frequency which has become a free vibration as shown in FIG. 9, and the kinetic spring constant K per unit area is calculated from the mean value of the so-read periods according to the following equation: ##EQU1## m=the mass of the load per unit area (250 Kg/m 3 ) Tn=the mean value (seconds) of the natural periods 
     60% Compression Set: According to the method of JIS K-6767. 
     Softening Point of Resin: ASTM D-1525 
     The properties referred to in the present invention are evaluated according to the following methods and ratings. 
     I. Shock-absorbing Property: 
     I-1. Shock-absorbing Property (1) (Compression Set): 
     Evaluation Method: According to JIS K-6767 (the compression creep test method), the 60% compression set is measured with respect to the thickness direction of the foam and two directions, that is, the longitudinal and lateral directions, rectangular to each other on the same plane and rectangular to the thickness direction, and the evaluation is made based on the minimum value (C). 
     Evaluation Rating: 
     
         ______________________________________                                    
60% Compression Set (C) (%)                                               
                     Mark                                                 
______________________________________                                    
15 ≧ C        ⊚                                     
15 &lt; C ≦ 25   ○                                             
25 &lt; C               X                                                    
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     I-2. Shock-absorbing Property (2) (Kinetic Shock-absorbing Characteristic): 
     Evaluation Method: According to the method of JIS Z-0235 (evaluated under a static stress of at least 0.02 Kg/cm 2 ), the measurement is carried out at a thickness of 50 mm (if the thickness is smaller than 50 mm, test pieces are piled so that the thickness is increased to 50 mm), and the maximum deceleration rate (G) is determined when the falling is conducted 2 to 5 times, and the evaluation is made based on the mean value of this maximum deceleration rate. The case where the static stress is smaller than 0.02 Kg/cm 2  is indicated by the mark (X). 
     Evaluation Rating: 
     
         ______________________________________                                    
Maximum Deceleration Rate (G)                                             
                      Mark                                                
______________________________________                                    
40 ≧ G         ⊚                                    
40 &lt; G ≦ 60    ○                                            
60 &lt; G                X                                                   
______________________________________                                    
 
    
     I-3. Shock-absorbing Property (3) [Filling Processability (low compression stress characteristic)]: 
     Evaluation Method: The stress under a 10% compression strain is measured with respect to the thickness direction of the foam and the two directions, that is, the longitudinal and lateral directions, rectangular to each other on the same plane and rectangular to the thickness direction, according to JIS K-6767 (the compression creep test method), and the evaluation is made based on the minimum value (P) among the values obtained with respect to the three directions. 
     Evaluation Rating: 
     
         ______________________________________                                    
Compression Stress (Kg/cm.sup.2)                                          
under 10% Strain     Mark                                                 
______________________________________                                    
1 ≧ P         ⊚                                     
1 &lt; P ≦ 2     ○                                             
2 &lt; P                X                                                    
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     I-4. Bending Deflection: 
     Evaluation Method: According to the method of JIS A-9511, the test is carried out, and the evaluation is made based on the maximum deflection (y) determined according to the following equation: ##EQU2## wherein, P: maximum load (Kg) 
     l: span distance (cm) 
     b: width of the test piece (cm) 
     h: thickness of the test piece (cm) 
     E: flexural modulus (Kg/cm 2 ) 
     y: maximum deflection (cm) 
     Evaluation Rating: 
     
         ______________________________________                                    
Maximum Deflection                                                        
                  Mark                                                    
______________________________________                                    
20 ≦ y     ⊚                                        
20 &gt; y ≧ 10                                                        
                  ○                                                
10 &gt; y            X                                                       
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     II. Heat Insulating Properties: 
     II-1. Heat Conductivity: 
     Evaluation Method: The heat conductivity (λ) at 0° C. is determined in unit of Kcal/m·hr°C. according to the method of ASTM C-518 and the evaluation is made based on the obtained value. 
     Evaluation Rating: 
     
         ______________________________________                                    
Heat Conductivity (Kcal/m · hr °C.)                       
                       Mark                                               
______________________________________                                    
0.033 ≧ λ                                                   
                       ⊚                                   
0.033 &lt; λ ≦ 0.043                                           
                       ○                                           
0.043 &lt; λ       X                                                  
______________________________________                                    
 
    
     II-2. Temperature Gradient of Heat Conductivity: 
     Evaluation Method: In the same manner as in the case of II-1, according to the method of ASTM C-518, the heat conductivity is measured at two temperatures or more (15° C., 35° C. and 55° C. in the present invention), and the temperature gradient (X) of the heat conductivity is determined and the evaluation is made based on this temperature gradient. 
     Evaluation Rating: 
     
         ______________________________________                                    
Temperature Gradient of                                                   
Heat Conductivity    Mark                                                 
______________________________________                                    
2.7 × 10.sup.-4 ≧ X                                          
                     ⊚                                     
2.7 × 10.sup.-4 &lt; X ≦ 3.3 × 10.sup.-4                  
                     ○                                             
3.3 × 10.sup.-4 &lt; X                                                 
                     X                                                    
______________________________________                                    
 
    
     III. Sound-insulating Property: 
     Evaluation Method: The measurement is carried out according to the floor shock sound level measuring method of JIS A-1418, and the evaluation is made according to the rating of the sound-insulating property to the floor shock sound, specified in JIS A-1419. A test specimen is fabricated by laying a plate-like foam having a thickness of 50 mm without any clearance on a building floor body of RC construction, applying a polyethylene film having a thickness of 100 microns onto the foam, applying a concrete mortar in a thickness of 50 mm onto the polyethylene film, aging for one week and placing a needle-punched carpet having a thickness of 3 mm on the concrete mortar, and the floor shock sound level is determined. 
     Evaluation Rating: 
     
         ______________________________________                                    
Class of Sound-insulating Effect                                          
                        Mark                                              
______________________________________                                    
above level L-50        ⊚                                  
below level L-50 but above level L-60                                     
                        ○                                          
below level L-60        X                                                 
______________________________________                                    
 
    
    
    
     EXAMPLE 1 AND COMPARATIVE EXAMPLE 1 
     Polystyrene extruded foams having a thickness of 25 mm, a width of 400 mm, a length of 700 mm and a density of 22 to 28 Kg/m 3  were placed in a heating furnace, and the heating medium, heating temperature and heating time were changed as indicated in Table 1-1 while keeping the aging conditions constant. Namely, in each run, the aging was carried out in air maintained at 75° C. for 120 hours. The structures of the obtained foams and the evaluation results are summarized in Table 1-2 and Table 1-3. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                 Example                  Comparative Example             
Sample foam No.  1   2    3   4   5   6   7   8    9   10  11             
__________________________________________________________________________
Kind of resin of used foam                                                
                 poly-                                                    
                     poly-                                                
                          poly-                                           
                              poly-                                       
                                  poly-                                   
                                      poly-                               
                                          poly-                           
                                              poly-                       
                                                   poly-                  
                                                       poly-              
                                                           poly-          
[Softening point (°C.)]                                            
                 styrene                                                  
                     styrene                                              
                          styrene                                         
                              styrene                                     
                                  styrene                                 
                                      styrene                             
                                          styrene                         
                                              styrene                     
                                                   styrene                
                                                       styrene            
                                                           styrene        
                 (105)                                                    
                     (105)                                                
                          (105)                                           
                              (105)                                       
                                  (105)                                   
                                      (105)                               
                                          (105)                           
                                              (105)                       
                                                   (105)                  
                                                       (105)              
                                                           (105)          
Preparation conditions                                                    
Blowing conditions                                                        
Heating medium   water                                                    
                     water                                                
                          water                                           
                              hot hot hot water                           
                                              water                       
                                                   hot hot hot            
                 vapor                                                    
                     vapor                                                
                          vapor                                           
                              water                                       
                                  water                                   
                                      water                               
                                          vapor                           
                                              vapor                       
                                                   water                  
                                                       air air            
Heating temperature (°C.)                                          
                 100 110  85  85  90  90  83  100  80  90  80             
Heating time (minutes)                                                    
                 10   1   60  60   1   5  60  0.95 0.5 10  30             
Ratio (%) of volume of once-                                              
                 37  70   64  44  58  55  80  84   90  68  110            
contracted foam to volume of                                              
foam after completion of aging                                            
Aging conditions                                                          
Temperature (°C.)                                                  
                 75  75   75  75  75  75  75  75   75  75  75             
Time (hours)     120 120  120 120 120 120 120 120  120 120 120            
Structure of obtained foam                                                
Density of foam (Kg/m.sup.3)                                              
                 10.0                                                     
                     15.0 13.0                                            
                              9.2 17.0                                    
                                      14.0                                
                                          17.7                            
                                              18.5 25.0                   
                                                       18.0               
                                                           24.2           
Wrinkles extending to center                                              
                 present                                                  
                     present                                              
                          present                                         
                              present                                     
                                  present                                 
                                      present                             
                                          present                         
                                              absent                      
                                                   absent                 
                                                       present            
                                                           absent         
of cell wall from joint                                                   
portion of cell wall                                                      
Average cell diameter of foam                                             
                 1.00                                                     
                     0.75 0.8 1.05                                        
                                  0.8 0.75                                
                                          0.78                            
                                              0.72 0.70                   
                                                       0.83               
                                                           0.70           
(mm)                                                                      
Closed-cell ratio (%)                                                     
                 82  89   92  90  95  90  92  92   98  89  92             
Kinetic spring constant                                                   
                 6.5 34.2 23.0                                            
                              30.0                                        
                                  40.0                                    
                                      37.0                                
                                          53.0                            
                                              58.0 100 55.3               
                                                           78.2           
(10.sup.6 N/m.sup.3)                                                      
Evaluation items                                                          
Shock-absorbing property (1) (%)                                          
                 15  23   23  14  25  21  27  30   34  24  32             
                 ⊚                                         
                     ○                                             
                          ○                                        
                              ⊚                            
                                  ○                                
                                      ○                            
                                          X   X    X   ○           
                                                           X              
Shock-absorbing property (2)                                              
                 ⊚                                         
                     ⊚                                     
                          ⊚                                
                              ⊚                            
                                  ○                                
                                      ⊚                    
                                          ○                        
                                              ○                    
                                                   X   ○           
                                                           ○       
Shock-absorbing property (3)                                              
                 ⊚                                         
                     ○                                             
                          ○                                        
                              ⊚                            
                                  ○                                
                                      ○                            
                                          ○                        
                                              X    X   ○           
                                                           X              
Bending deflection                                                        
                 ⊚                                         
                     ○                                             
                          ○                                        
                              ○                                    
                                  ○                                
                                      ○                            
                                          X   X    X   X   X              
Heat insulating property (1)                                              
                 ⊚                                         
                     ⊚                                     
                          ⊚                                
                              ○                                    
                                  ⊚                        
                                      ⊚                    
                                          ⊚                
                                              ⊚            
                                                   ⊚       
                                                       ○           
                                                           X              
Heat insulating property (2)                                              
                 ○                                                 
                     ○                                             
                          ⊚                                
                              ○                                    
                                  ⊚                        
                                      ○                            
                                          ○                        
                                              ⊚            
                                                   ⊚       
                                                       ○           
                                                           .circleincircle
                                                           .              
Sound insulating property                                                 
                 ⊚                                         
                     ○                                             
                          ⊚                                
                              ○                                    
                                  ○                                
                                      ○                            
                                          X   X    X   X   X              
__________________________________________________________________________
 
    
     EXAMPLE 2 AND COMPARATIVE EXAMPLE 2 
     Extruded foams having a thickness of 15 mm, a width of 300 mm, a length of 500 mm and a density of 25 to 110 Kg/m 3  were placed in a heating furnace as in Example 1 and Comparative Example 1. The heating medium was limited to steam or hot water, and the heating temperature was adjusted to 90° or 100° C. and the heating was conducted for 15 or 30 minutes. The same aging conditions were adopted in all the runs (See Table 2-1). The ratio of the volume of the once-contracted foam to the volume of the foam after completion of the aging was examined. The structures of the obtained foams and the evaluation results are summarized in Table 2-2 and Table 2-3. As shown in Table 2-1, the used resins were polystyrene, polymethyl methacrylate and a styrene-methacrylic acid copolymer. 
     
                                           TABLE 2                                 
__________________________________________________________________________
                 Example                   Comparative Example            
Sample foam No.  12   13  14     15  16    17  18    19  20               
__________________________________________________________________________
Kind of resin of used foam                                                
                 poly-                                                    
                      poly-                                               
                          polymethyl                                      
                                 poly-                                    
                                     styrene/                             
                                           poly-                          
                                               styrene/                   
                                                     poly-                
                                                         styrene/         
[Softening point (°C.)]                                            
                 styrene                                                  
                      styrene                                             
                          methacrylate                                    
                                 styrene                                  
                                     methacrylic                          
                                           styrene                        
                                               methacrylic                
                                                     styrene              
                                                         methacrylic      
                 (105)                                                    
                      (105)                                               
                          (103)  (105)                                    
                                     acid  (105)                          
                                               acid  (105)                
                                                         acid             
                                     copolymer copolymer copolymer        
                                     (125)     (125)     (125)            
Preparation conditions                                                    
Blowing conditions                                                        
Heating medium   water                                                    
                      water                                               
                          water vapor                                     
                                 hot hot water                            
                                           water                          
                                               water hot hot water        
                 vapor                                                    
                      vapor      water     vapor                          
                                               vapor water                
Heating temperature (°C.)                                          
                 100  100 100    90  90    100 100   90  90               
Heating time (minutes)                                                    
                 15   15  15     30  30    15  15    30  30               
Ratio (%) of volume of once-                                              
                 20   30  36     62  70    72  80    105 95               
contracted foam to volume of                                              
foam after completion of aging                                            
Aging conditions                                                          
Temperature (°C.)                                                  
                 85   85  85     85  85    85  85    85  85               
Time (hours)     96   96  96     96  96    96  96    96  96               
Structure of obtained foam                                                
Density of foam (Kg/m.sup.3)                                              
                 3.3  5.0 8.5    7.2 15.1  16.3                           
                                               17.3  26.0                 
                                                         28.0             
Wrinkles extending to center                                              
                 present                                                  
                      present                                             
                          present                                         
                                 present                                  
                                     present                              
                                           present                        
                                               present                    
                                                     absent               
                                                         absent           
of cell wall from joint                                                   
portion of cell wall                                                      
Average cell diameter of foam                                             
                 1.85 1.62                                                
                           1.12   0.42                                    
                                      0.9  0.32                           
                                               0.77  0.83                 
                                                         0.72             
(mm)                                                                      
Closed-cell ratio (%)                                                     
                 50   53  62     68  92    23  48    60  90               
Kinetic spring constant                                                   
                 1.7  2.4 5.3    5.2 15.5  20.5                           
                                               45.1  230 130              
(10.sup.6 N/m.sup.3)                                                      
Evaluation items                                                          
Shock-absorbing property (1) (%)                                          
                 5.7  8.0 10.2   7.7 24     8.5                           
                                               10.0  25  40               
                 ⊚                                         
                      ⊚                                    
                          ⊚                                
                                 ⊚                         
                                     ○                             
                                           ⊚               
                                               ⊚           
                                                     X   X                
Shock-absorbing property (2)                                              
                 ⊚                                         
                      ⊚                                    
                          ⊚                                
                                 ⊚                         
                                     ○                             
                                           ⊚               
                                               ⊚           
                                                     ○             
                                                         X                
Shock-absorbing property (3)                                              
                 ⊚                                         
                      ⊚                                    
                          ⊚                                
                                 ⊚                         
                                     ○                             
                                           ⊚               
                                               ⊚           
                                                     ○             
                                                         X                
Bending deflection                                                        
                 ⊚                                         
                      ⊚                                    
                          ⊚                                
                                 ⊚                         
                                     ○                             
                                           ⊚               
                                               ○                   
                                                     X   X                
Heat insulating property (1)                                              
                 ○                                                 
                      ○                                            
                          ○                                        
                                 ○                                 
                                     ⊚                     
                                           X   X     ○             
                                                         ⊚ 
                                                         1                
Heat insulating property (2)                                              
                 ○                                                 
                      ○                                            
                          ○                                        
                                 ○                                 
                                     ⊚                     
                                           X   X     ○             
                                                         ⊚ 
                                                         7                
Sound insulating property                                                 
                 ⊚                                         
                      ⊚                                    
                          ⊚                                
                                 ⊚                         
                                     ⊚                     
                                           ⊚               
                                               ○                   
                                                     X   X                
__________________________________________________________________________
 
    
     EXAMPLE 3 AND COMPARATIVE EXAMPLE 3 
     Polystyrene extruded foams having a density of 22 to 28 Kg/m 3  and having the same size as that of the samples used in Example 1 and Comparative Example 1 were heated and blown under limited conditions (heating medium, heating temperature and heating time) and aged under various conditions (temperature and time) (See Table 3-1). The structures of the obtained foams and the evaluation results are summarized in Table 3-2 and Table 3-3. The used polystyrenes were two kinds differing in the softening point. 
     
                                           TABLE 3                                 
__________________________________________________________________________
               Example                     Comparative Example            
Sample foam No.                                                           
               1   21  22  23  24  25  26  27  28  29  30  31             
__________________________________________________________________________
Kind of resin of used foam                                                
               poly-                                                      
                   poly-                                                  
                       poly-                                              
                           poly-                                          
                               poly-                                      
                                   poly-                                  
                                       poly-                              
                                           poly-                          
                                               poly-                      
                                                   poly-                  
                                                       poly-              
                                                           poly-          
[Softening point (°C.)]                                            
               styrene                                                    
                   styrene                                                
                       styrene                                            
                           styrene                                        
                               styrene                                    
                                   styrene                                
                                       styrene                            
                                           styrene                        
                                               styrene                    
                                                   styrene                
                                                       styrene            
                                                           styrene        
               (105)                                                      
                   (105)                                                  
                       (105)                                              
                           (112)                                          
                               (112)                                      
                                   (112)                                  
                                       (112)                              
                                           (105)                          
                                               (105)                      
                                                   (112)                  
                                                       (112)              
                                                           (112)          
Preparation conditions                                                    
Blowing conditions                                                        
Heating medium water                                                      
                   water                                                  
                       hot water                                          
                               water                                      
                                   hot hot water                          
                                               water                      
                                                   water                  
                                                       hot hot            
               vapor                                                      
                   vapor                                                  
                       water                                              
                           vapor                                          
                               vapor                                      
                                   water                                  
                                       water                              
                                           vapor                          
                                               vapor                      
                                                   vapor                  
                                                       water              
                                                           water          
Heating temperature (°C.)                                          
               100 100 90  110 110 90  100 100 100 110 90  90             
Heating time (minutes)                                                    
               10  10  20   5   5  20  20  10  10   5  20  20             
Ratio (%) of volume of once-                                              
               37  60  52  64  67  70  68  72  69  90  93  98             
contracted foam to volume of                                              
foam after completion of aging                                            
Aging conditions                                                          
Temperature (°C.)                                                  
               75  85  75  105 110 60  75  75  100 115 55  58             
Time (hours)   120 24  96  48  24  24  72  23  15  24  120 23             
Structure of obtained foam                                                
Density of foam (Kg/m.sup.3)                                              
                10.0                                                      
                   15.1                                                   
                       12.7                                               
                           13.0                                           
                               11.8                                       
                                   16.8                                   
                                       13.0                               
                                           17.1                           
                                               17.6                       
                                                   25.3                   
                                                       19.8               
                                                           23.2           
Wrinkles extending to                                                     
               present                                                    
                   present                                                
                       present                                            
                           present                                        
                               present                                    
                                   present                                
                                       present                            
                                           present                        
                                               present                    
                                                   absent                 
                                                       present            
                                                           present        
center of cell wall from                           (dis-                  
joint portion of cell wall                         solved)                
Average cell diameter of                                                  
               1.0  2.0                                                   
                       0.93                                               
                           0.75                                           
                               1.3  1.5                                   
                                        1.7                               
                                           0.82                           
                                               1.72                       
                                                   mea-                   
                                                       1.65               
                                                            2.1           
foam (mm)                                          sure-                  
Closed-cell ratio (%)                                                     
               82  52  90  62  50  93  92  93  88  ment                   
                                                       91  90             
Kinetic spring constant                                                   
               6.5 20.1                                                   
                       34.4                                               
                           21.8                                           
                               7.3 40.0                                   
                                       38.0                               
                                           42.0                           
                                               39.7                       
                                                   impos-                 
                                                       88.3               
                                                           130            
(10.sup.6 N/m.sup.3)                               sible                  
Evaluation items                                                          
Shock-absorbing property (1)                                              
               15  10.5                                                   
                       22  17.0                                           
                               7.0 24  23  26  26  --  30  31             
(%)            ⊚                                           
                   ⊚                                       
                       ○                                           
                           ○                                       
                               ⊚                           
                                   ○                               
                                       ○                           
                                           X   X   --  X   X              
Shock-absorbing property (2)                                              
               ⊚                                           
                   ⊚                                       
                       ○                                           
                           ⊚                               
                               ⊚                           
                                   ○                               
                                       ○                           
                                           ○                       
                                               ○                   
                                                   --  X   X              
Shock-absorbing property (3)                                              
               ⊚                                           
                   ⊚                                       
                       ⊚                                   
                           ⊚                               
                               ⊚                           
                                   ○                               
                                       ○                           
                                           ○                       
                                               X   --  X   X              
Bending deflection                                                        
               ⊚                                           
                   ○                                               
                       ○                                           
                           ○                                       
                               ⊚                           
                                   ○                               
                                       ○                           
                                           X   X   --  X   X              
Heat insulating property (1)                                              
               ○                                                   
                   ○                                               
                       ⊚                                   
                           ⊚                               
                               ○                                   
                                   ○                               
                                       ○                           
                                           ⊚               
                                               ○                   
                                                   --  ○           
                                                           X              
Heat insulating property (2)                                              
               ○                                                   
                   ○                                               
                       ⊚                                   
                           ○                                       
                               ○                                   
                                   ⊚                       
                                       ⊚                   
                                           ⊚               
                                               ○                   
                                                   --  ⊚   
                                                           ○       
Sound insulating property                                                 
               ⊚                                           
                   ⊚                                       
                       ○                                           
                           ⊚                               
                               ⊚                           
                                   ○                               
                                       ○                           
                                           X   ○                   
                                                   --  X   X              
__________________________________________________________________________
 
    
     EXAMPLE 4 AND COMPARATIVE EXAMPLE 4 
     In order to clarify that the foam of the present invention possesses all of the properties referred to in the present invention and where the foam of the present invention is characteristic over commercially available products, the following foamed plates were evaluated according to the evaluation methods described in the text. The evaluation results are summarized in Table 4-1 and Table 4-2. 
     Foams (representatives) of Present Invention: 
     Samples Nos. 1, 15, and 22 
     Commercially Available Products: 
     
         ______________________________________                                    
Mark  Name       Main Use        Maker                                    
______________________________________                                    
S     Glass Wool heat-insulating material,                                
                                 S Company                                
      No. 2      sound-insulating material                                
W     Woodrack   packaging material                                       
                                 Asahi-Dow Ltd.                           
                 (fancy box)                                              
X     Styrofoam  heat-insulating material                                 
                                 Asahi-Dow Ltd.                           
Y     Beads Board                                                         
                 heat-insulating material                                 
                                 Y Company                                
      No. 2                                                               
Z     Beads Board                                                         
                 shock-absorbing material                                 
                                 Z Company                                
      No. 3                                                               
______________________________________                                    
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
             Structure of Foam                                            
                    Wrinkles extending                                    
             Density of                                                   
                    to center of cell wall                                
                               Average cell  Kinetic spring               
       Material                                                           
             product                                                      
                    from joint portion                                    
                               diameter                                   
                                      Closed-cell                         
                                             constant                     
       of product                                                         
             (Kg/m.sup.3)                                                 
                    of cell wall                                          
                               (mm)   ratio (%)                           
                                             (10.sup.6 N/m.sup.3)         
__________________________________________________________________________
Product                                                                   
       poly- 10.0   present    1.0    82     6.5                          
No. 1 of                                                                  
       styrene                                                            
present                                                                   
invention                                                                 
Product                                                                   
       poly-  7.2   present    0.42   68     5.2                          
No. 15 of                                                                 
       styrene                                                            
present                                                                   
invention                                                                 
Product                                                                   
       poly- 12.7   present    0.93   90     34.4                         
No. 22 of                                                                 
       styrene                                                            
present                                                                   
invention                                                                 
Commercial                                                                
       glass 20.0   --         --     --     1.0                          
product S                                                                 
       wool                                                               
Commercial                                                                
       poly- 100.0  absent     0.40   95     130                          
product W                                                                 
       styrene                                                            
Commercial                                                                
       poly- 28.0   absent     0.75   98     110                          
product X                                                                 
       styrene                                                            
Commercial                                                                
       poly- 30.0   absent     0.2    90     120                          
product Y                                                                 
       styrene                                                            
Commercial                                                                
       poly- 15.0   absent     0.3    83     65                           
product Z                                                                 
       styrene                                                            
__________________________________________________________________________
       Evaluation Items                                                   
       Shock-absorbing                                                    
       property  Shock-                                                   
                       Shock-    Heat-    Heat-                           
       (1). mean value (%)                                                
                 absorbing                                                
                       absorbing insulating                               
                                          insulating                      
       of 60% compression                                                 
                 property                                                 
                       property                                           
                            Bending                                       
                                 property property                        
                                               Sound-                     
       set values 35 CH                                                   
                 (2)   (3)  deflec-                                       
                                 (1)      (2)  insulating                 
       in three directions                                                
                 (Value G)                                                
                       (Kg/cm.sup.2)                                      
                            tion (Kcal/m · hr °C.)        
                                          (× 10.sup.-4)             
                                               property                   
__________________________________________________________________________
Product                                                                   
       15        38    0.7  22   0.033    2.8  ⊚           
No. 1 of                                                                  
       ⊚                                                   
                 ⊚                                         
                       ⊚                                   
                            ⊚                              
                                 ⊚                         
                                          ○                        
present                                                                   
invention                                                                 
Product                                                                   
       7.7       35    0.5  30   0.035    3.0  ⊚           
No. 15 of                                                                 
       ⊚                                                   
                 ⊚                                         
                       ⊚                                   
                            ⊚                              
                                 ○ ○                        
present                                                                   
invention                                                                 
Product                                                                   
       22        44    0.9  12   0.032    2.4  ○                   
No. 22 of                                                                 
       ○  ○                                                 
                       ⊚                                   
                            ○                                      
                                 ⊚                         
                                          ⊚                
present                                                                   
invention                                                                 
Commercial                                                                
       3.0       --    --   --   0.038    100  ⊚           
product S                                                                 
       ⊚                                                   
                 X     ⊚                                   
                            ⊚                              
                                 ○ X                               
Commercial                                                                
       42        --    5.0   2   0.032    2.5  X                          
product W                                                                 
       X         X     X    X    ⊚                         
                                          ⊚                
Commercial                                                                
       38        75    3.5  3.8  0.029    1.3  X                          
product X                                                                 
       X         X     X    X    ⊚                         
                                          ⊚                
Commercial                                                                
       40        80    3.5  3.2  0.029    1.7  X                          
product Y                                                                 
       X         X     X    X    ⊚                         
                                          ⊚                
Commercial                                                                
       27        70    2.4  5.3  0.034    2.9  X                          
product Z                                                                 
       X         X     X    X    ⊚                         
                                          ○                        
__________________________________________________________________________
 
    
     By dint of the above-mentioned characteristic structure, the present invention allows the foam to have a high flexibility, an excellent heat-insulating property, a good sound-insulating property and a reduced compression set, and the foam of the present invention can advantageously be used as a heat-insulating and sound-insulating material or a shock-absorbing material for a floor, wall or roof of buildings. Accordingly, the present invention is very advantageous from the industrial viewpoint. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1-(a) is an enlarged photograph showing the cellular structure of the product of the present invention. FIG. 1-(b) is an enlarged photograph showing the cellular structure of a comparative product. FIG. 2 is a diagram showing an example of the floating floor construction method. FIG. 3 is a graph showing the state of reduction of the density of the foam by heating and blowing. FIG. 4 is a graph showing the state of reduction of the density in various heating media. FIG. 5-(a) is an enlarged photograph showing the cellular structure of the product of the present invention. FIG. 5-(b) is an enlarged photograph showing the cellular structure of a comparative product. FIG. 6 is a graph showing the recovery state of the foam at various aging temperatures. FIG. 7 is a graph showing an example of the temperature dependency of permeation of water vapor through a polystyrene wall. FIG. 8 and FIG. 9 are diagrams illustrating the method for measuring the kinetic spring constant.