Patent Publication Number: US-4546127-A

Title: Low-resilience rubber compositions and foams

Description:
This application is a continuation of application Ser. No. 484,510 filed on Apr. 13, 1983. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to rubber and foamed rubber compositions which retain low resilience over a wide temperature range of from -10° C. to 50° C. and are soft at room temperature. More particularly, this invention relates to rubber and foamed rubber compositions which comprise a styrene-butadiene copolymer or a mixture of styrene-butadiene copolymers having a high styrene content, and relatively large amounts of a filler and a plasticizer, and which have improved vibration-attenuating and shock-absorbing properties at frequencies of 0.01-1000 Hz due to their remarkably low hardness, improved energy- and shock-absorbing ability due to their low resilience over a wide temperature range from -10° C. to 50° C., and an increased tensile strength. 
     A number of rubber compositions which are soft at room temperature are known in the art, including compositions based on polynorbornene as disclosed in Japanese Patent Application Kokai No. 56-74801, compositions based on ethylene-vinyl acetate copolymers as disclosed in Japanese Patent Publication No. 53-3244, and compositions based on urethane elastomers as disclosed in Japanese Patent Application Kokai No. 52-54000. These compositions, however, were inadequate for practical application because some had a resilience as high as 40 to 90% or a reduced tensile strength of less than 50 kg/cm 2 , and none of them satisfied both low resilience and high tensile strength. Low-resilience rubber compositions were also proposed which were based on butyl rubber or modified rubbers. However, they were temperature dependent and had increased hardness. 
     Most conventional sponge rubbers using organic foaming agents are highly resilient. As far as the inventors know, few investigations have been made on low-resilience sponge rubbers. As to foamed materials, some of foamed polyurethanes have low resilience (see Japanese Patent Application Kokai No. 55-137123, and Japanese Patent Publication Nos. 46-34630, 48-38467, and 50-52175, for example), although they have poor shock-absorbing properties and often become unstable with temperature change. In addition, these foamed polyurethanes are known to be inferior in water resistance and heat deterioration to sponge rubbers, and they are also unsatisfactory in actual application. 
     The current general trend in the rubber industry is toward light weight products. For low-resilience rubber compositions, it is desired to render them light weight to make them more valuable. However, simply foaming conventional low-resilience rubber compositions with the aid of an organic foaming agent will result in foams having increased resilience because the resilience is largely affected by the introduced closed cells containing gases. 
     SUMMARY OF THE INVENTION 
     Undertaking expensive research for the purpose of obtaining a rubber composition which can meet the requirements of low hardness, low resilience, and high tensile strength at the same time, it has been found that a rubber composition comprising a styrene-butadiene copolymer or a mixture of styrene-butadiene copolymers having a preponderant styrene content, and specific amounts of a filler and a plasticizer meets the three requirements in a cured state. This rubber composition, when a foaming agent is further added, may be expanded, at no expense to the low-resilient, into a low-resilience light-weight rubber foam. 
     Accordingly, an object of the present invention is to provide a rubber composition characterized by a semi-liquid nature, low hardness and high mechanical strength, which make it suitable for use in acoustic parts such as turntable sheets, vibration-damping materials, shock-absorbing materials, earthquake-insulating materials, sound-insulating materials and the like. 
     It is another object of the present invention to provide a rubber foam characterized by a light-weight and semi-liquid nature which make it suitable for use in acoustic parts such as turntable sheets, vibration-damping material, shock-absorbing materials, earthquake-insulating materials, sound-insulating materials, various grips, insoles and soles of shoes, face sheets of table tennis rackets, baseball gloves (mitts), protectors, and helmets, and other gymnastic aids. 
     According to the present invention, there is provided a soft, low-resilient, cured rubber composition prepared by mixing 
     (a) 100 parts by weight of a polymeric material comprising a styrene-butadiene copolymer or a mixture of styrene-butadiene copolymers having a styrene content of 53 to 75% by weight of the copolymer or copolymer mixture, and 0 to 40% by weight of the polymeric material of another rubber ingredient, 
     (b) 30 to 300 parts by weight of a filler, and 
     (c) 5 to 100 parts by weight of a plasticizer, adding effective amounts of a vulcanizing agent and a vulcanizing accelerator to the mixture, and then heat curing the mixture. 
     This cured composition is characterized by a hardness of 40° or lower as measured by a type-A hardness tester as specified by the Japanese Industrial Standard, JIS 6301, a resilience of 30% or lower over a temperature range from -10° C. to 50° C., and a tensile strength of 50 kg/cm 2  or higher, and these properties make it suitable for use in the above-mentioned applications. 
     The present invention also provides a rubber foam which is a foamed modification of the above-defined composition. According to a second aspect of the present invention, there is provided a low-resilience rubber foam prepared by mixing 
     (a) 100 parts by weight of a polymeric material comprising a styrene-butadiene copolymer or a mixture of styrene-butadiene copolymers having a styrene content of 53 to 75% by weight of the copolymer or copolymer mixture, and 0 to 40% by weight of the polymeric material of another rubber ingredient, 
     (b) 30 to 300 parts by weight of a filler, 
     (c) 5 to 100 parts by weight of a plasticizer, and 
     (d) 0.5 to 60 parts by weight of a foaming agent, adding effective amounts of a vulcanizing agent and a vulcanizing accelerator to the mixture, and heating the mixture for curing and foaming. 
     This rubber foam is made at little expense to the low-resilience performance of the rubber composition of the invention. More particularly, the foamed, cured rubber has improved energy-and shock-absorbing ability due to a low resilience of 35% or less in a service temperature range from -10° C. to 50° C., improved vibration-absorbing properties over a frequency range of 0.01 to 1000Hz due to low hardness, a low density as demonstrated by a bulk specific gravity of 0.7 or less, and a permanent compressive set of 5 to 50%. 
     The rubber and foamed rubber compositions of the present invention are particularly suitable for use in various components of shoes such as sock-linings, insoles, midsoles, inner cores, ankle pads, toe guards, and tongues. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent and understandable from the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a partially cut-away side elevation of a shoe illustrating its components to which the rubber composition of the invention is applicable, and 
     FIGS. 2, 3, and 4 are cross-sectional views of different laminates for use as a shoe insole, respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The polymeric material (a) used in the rubber and foamed rubber compositions of the invention comprises a styrene-butadiene copolymer or a mixture of styrene-butadiene copolymers and optionally, another rubber ingredient. The styrenebutadiene copolymer or styrene-butadiene copolymer mixture must have a styrene content of 53 to 75% by weight, preferably 56 to 59% by weight. The copolymer mixture is prepared by mixing styrene-butadiene copolymers having different styrene contents such that the styrene content of the mixture may fall within the above-specified range. In preparing the copolymer mixture, styrene-butadiene copolymers having styrene contents of 42 to 75% by weight may preferably be used. 
     The other rubber ingredient may preferably be a rubber having a glass transition temperature of -20° C. or lower. It may be selected from natural rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber, chloroprene rubber, acrylonitrile-butadiene rubber, ethylene-propylene rubber, and similar rubbers. These other rubber ingredients may be blended with the styrene-butadiene copolymer(s) in amounts of 0 to 40% by weight of the polymeric material. 
     The filler (b) is present in amounts of 30 to 300 parts by weight, preferably 150 to 200 parts by weight per 100 parts by weight of the polymeric material. The filler may be either a low reinforcing or a non-reinforcing filler. It may preferably be selected from calcium carbonate, barium sulfate, magnesium carbonate, dolomite, kaolin clay, calcined clay, hard clay, sericite, talc, wollastonite, bentonite, commonmica, aluminum hydroxide, zinc oxide, activated clay, halloysite, titanium oxide, gypsum, light calcium carbonate, heavy calcium carbonate, barium carbonate, strontium carbonate, diatomaceous earth, silas, silas balloon, calcium sulfate, etc. 
     The plasticizer (c) may be any suitable plasticizer commonly used with rubbers. It may be selected from aromatic oils, naphthenic oils, spindle oil, dioctyl phthalate, polybutene, phosphoric acid derivatives such as tricresyl phosphate, fatty acid derivatives, sulfonic acid derivatives, and the like. The plasticizer is present in amounts of 5 to 100 parts by weight per 100 parts by weight of the polymeric material. For rubber compositions, the plasticizer may preferably be present in amounts of 50 to 100 parts, especially 65 to 85 parts per 100 parts by weight of the polymeric material. For foamed rubber compositions, it may preferably be present in amounts of 5 to 60 parts, especially 20 to 40 parts per 100 parts by weight of the polymeric material. 
     Also included in the rubber and foamed rubber compositions of the present invention are additives commonly used in rubber formulation, for example, reinforcing agents such as carbon black and silica, and antioxidants for preventing aging such as wax. 
     Rubber compositions may be prepared according to the present invention by adding effective amounts of a vulcanizing agent and a vulcanizing accelerator to a mixture of the above-defined components (a), (b), and (c) and optional additives, and heat curing the resulting mixture. The vulcanizing agent may be any well-known vulcanizing agent, for example, organic peroxides, sulfur, and organic sulfur compounds such as tetramethyl thiuram disulfide, and is added in amounts of 0.5 to 10 parts by weight per 100 parts by weight of the polymeric material. Also the vulcanizing accelerator may be any well-known accelerator selected from aldehydeamines, aldehydeammonias, dithiocarbamates, guanidines, thiurams, sulfenamides, thiazoles, thioureas, and the like, and is added in amounts of 0.5 to 10 parts by weight per 100 parts by weight of the polymeric material. 
     The rubber compositions of the present invention may be mixed and cured in a conventional manner. The curing temperature may range from 110° C. to 180° C. Curing temperatures lower than 110° C. require a long time to complete curing whereas temperatures higher than 180° C. will result in cured rubber compositions having reduced physical properties. 
     Foamed rubber compositions may be obtained according to the present invention simply by using a foaming agent (d) in combination with the above-defined components (a), (b), and (c). The foaming agent may be any suitable foaming agent commonly used in rubber expansion and selected from organic foaming agents such as dinitrosopentamethylenetetramine (DPT), azodicarbonamide (AZC), p-toluenesulfonyl hydrazide (TSH), 4,4&#39;-oxybisbenzenesulfonyl hydrazide (OBSH), etc., and inorganic foaming agents such as sodium hydrogen carbonate. The foaming agent is present in amounts of 0.5 to 60 parts, preferably 2 to 30 parts by weight per 100 parts by weight of the polymeric material. Low-resilience foamed rubbers may be obtained by adding effective amounts of a vulcanizing agent and a vulcanizing accelerator as mentioned above to a mixture of the above-defined four components (a)-(d) and optional additives and heating the resulting mixture for foaming and curing. In this case, the rubber compositions may be mixed and cured in a conventional manner. The curing temperature may range from 110° to 180° C. Curing temperatures of lower than 110° C. will be too low to complete curing within an acceptable time and to fully decompose the foaming agent. Temperatures higher than 180° C. will result in foamed, cured rubber compositions which have reduced physical properties and are unstable. 
     The rubber and foamed rubber compositions of the invention have improved shock-absorbing properties which make them suitable in a variety of applications, particularly as shoe-making materials. More specifically, the compositions of the invention are suitable for manufacturing shoes capable of protecting foot joints and nervous systems by absorbing excessive impact force occurring on walking and running. They are also suitable to make various parts of shoes such as gymnastic shoes and medical shoes, for example, as shown in FIG. 1, a shoe-lining 1, insole 2, mid-sole 3, inner core 4, ankle pad 5, toe guard 6, tongue 7, outer sole 8, upper 9, and arch support 10. In the manufacture of insoles, making them from the composition of the invention alone, of course, provides satisfactory performance although insoles of better performance may be obtained by assembling the composition of the invention with another rubber material into a laminate. As shown in FIGS. 2-4, a layer 11 of the present rubber composition is attached to another layer 12 of an ethylene-vinyl acetate copolymer, polyurethane foam, rubber foam, or rubber composition based on generalpurpose rubbers, resulting in a laminate which has enhanced shock-absorbing and cushioning properties. The generalpurpose rubbers used in combination may be selected from natural rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber, chloroprene rubber, acrylonitrile-butadiene rubber, ethylene-propylene rubber and other rubbers as long as they have a glass transition temperature of lower than -20° C. The rubber foams include both foamed materials prepared by vulvanizing and expanding organic or inorganic foaming agent-containing rubbers as well as so-called foam rubber prepared from latex. In FIGS. 2-4, numeral 13 designates a fabric lining attached to the top surface of the layer 12. 
     Following are examples of the present invention, which are set forth by way of illustration and not by way of limitation. All parts are by weight. cl EXAMPLES 1-8 AND COMPARATIVE EXAMPLES 1-4 
     Cured samples were prepared by concurrently mixing and milling 100 parts of styrene-butadiene copolymers having different styrene contents as shown in Table 1, 150-200 parts of a filler, and 50-80 parts of a plasticizer, and adding a vulcanizing agent and a vulcanizing accelerator to the mixture, followed by heat curing. 
     These samples were determined for hardness, resilience, and tensile properties. The results are shown in Table 1.  Hardness measurement was made using a type A hardness tester as specified in JIS 6301, and resilience measurement was made using a Dunlop tripsometer. Tensile properties, namely, tensile strength Tb and elongation Eb were measured on DIN-3 type specimens using an Instron tensile tester at room temperature and a pulling rate of 100 mm/min. 
     For the sake of comparison, similar experiments were made using butyl rubber and urethane elastomer. The results are shown in Table 2. 
     
                                           TABLE 1                                 
__________________________________________________________________________
              Example                                                     
Composition, parts by weight                                              
              1   2   3   4   5   6   7   8                               
__________________________________________________________________________
Styrene-butadiene copolymer                                               
type 1*.sup.1 100 100 75  75  70  70  60  65                              
type 2*.sup.2 --  --  25  25  30  25  30  30                              
type 3*.sup.3                     5   10  15                              
Filler (BaSO.sub.4)                                                       
              150 200 150 200 150 200 200 200                             
Plasticizer (aromatic oil)                                                
              70  70  70  80  80  80  80  80                              
Zinc oxide    5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0                             
Stearic acid  3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0                             
Sulfur        1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5                             
Accelerator A*.sup.4                                                      
              1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0                             
Accelerator B*.sup.5                                                      
              0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5                             
Properties                                                                
Hardness at 20° C.                                                 
              29  36  26  29  28  25  29  29                              
Resilience at -10° C., %                                           
              21.6                                                        
                  17.8                                                    
                      16.4                                                
                          14.9                                            
                              14.9                                        
                                  12.0                                    
                                      10.1                                
                                          12.3                            
Resilience at 020° C., %                                           
                  2.2 0.09                                                
                          1.0 1.1 0.09                                    
                                      0   0                               
Resilience at 19.3degree. C., %                                           
                  14.2                                                    
                      15.7                                                
                          14.6                                            
                              14.6                                        
                                  15.0                                    
                                      14.2                                
                                          13.1                            
Tb, kg/cm.sup.2                                                           
              58.6                                                        
                  57.8                                                    
                      59.7                                                
                          58.3                                            
                              58.6                                        
                                  52.3                                    
                                      53.1                                
                                          51.3                            
Eb, %         620 640 600 645 600 670 670 670                             
Styrene content, *.sup.6, %                                               
              65.0                                                        
                  65.0                                                    
                      66.8                                                
                          66.8                                            
                              67.1                                        
                                  66.3                                    
                                      66.1                                
                                          66.6                            
__________________________________________________________________________
 *.sup.1 styrene content 65 wt %;                                         
 *.sup.2 styrene content 72 wt %;                                         
 *.sup.3 styrene content 55 wt %                                          
 *.sup.4 2mercaptobenzothiazole;                                          
 *.sup.5 dio-tolyl-guanidine;                                             
 *.sup.6 based on the total weight of styrenebutadiene copolymers.        
 Vulcanizing conditions: 160° C., 40 minutes.                      
 
    
     
                       TABLE 2                                                     
______________________________________                                    
             Comparative Example                                          
Composition, parts                                                        
               1       2       3     4                                    
______________________________________                                    
Butyl rubber   100     100     100   com-                                 
Filler (BaSO.sub.4)                                                       
               150     --      --    mercially                            
Plasticizer (aromatic oil)                                                
               30      --      --    available                            
Carbon black   --      50.0    --    urethane                             
Zinc oxide     5.0     5.0     5.0   elastomer                            
Stearic acid   3.0     1.0     1.0                                        
Sulfur         2.0     2.0     2.0                                        
Accelerator B  0.5     --      --                                         
Accelerator C*.sup.7                                                      
               1.0     1.0     1.0                                        
Properties                                                                
Hardness at 20° C.                                                 
               26      62      36    8                                    
Resilience at -10° C., %                                           
               6.6     8.0     6.6   --                                   
Resilience at  20° C., %                                           
               17.6    24.0    46.6  20.0                                 
Resilience at  45° C., %                                           
               30.6    41.0    64.2  40.0                                 
Tb, kg/cm.sup.2                                                           
               20.5    201.6   19.8  1.4                                  
Eb, %          370     600     368   700                                  
______________________________________                                    
 *.sup.7 tetramethyl thiuram monosulfide                                  
 
    
     Comparison of Table 1 with Table 2 reveals that the rubber compositions of the present invention exhibit lower resilience over a wider temperature range, lower hardness, and higher strength than the conventional rubber compositions. 
     EXAMPLES 9-12 
     Compositions having formulations shown in Table 3 were milled for 30 minutes through a roll mill having a surface temperature of 60° C. into a sheet, placed in a mold cavity of 10 cm by 10 cm by 1 cm, heated to 140° C. under a pressure of 90 kg/cm 2  for 25 minutes, and then taken out of the mold. The foamed samples were determined for hardness, resilience, and tensile properties to give the data shown in Table 4. 
     For the sake of comparison, similarly processed samples of commercially available chloroprene rubber foam, natural rubber foam, EVA foam, and RB foam were also determined for the same physical properties to give the data shown in Table 5. 
     
                       TABLE 3                                                     
______________________________________                                    
               Example                                                    
Composition      9       10      11    12                                 
______________________________________                                    
Styrene-butadiene copolymer                                               
type 1*.sup.1    65      65      65    50                                 
type 2*.sup.2    30      30      30    45                                 
type 3*.sup.3    5       5       5     5                                  
Filler (BaSO.sub.4)                                                       
                 100     100     100   100                                
Plasticizer (aromatic oil)                                                
                 30      30      30    30                                 
Zinc oxide       5.0     5.0     5.0   5.0                                
Stearic acid     3.0     3.0     3.0   3.0                                
Carbon           1.0     1.0     1.0   1.0                                
Sulfur           2.1     2.1     2.1   2.1                                
Accelerator*.sup.8                                                        
                 0.8     0.8     0.8   0.8                                
Foaming agent*.sup.9                                                      
                 5       10      20    20                                 
______________________________________                                    
 *.sup.1,2,3 see Table 1                                                  
 *.sup.8 N--cyclohexyl2-benzothiazole sulfenamide                         
 *.sup.9 azodicarbonamide foaming agent                                   
 
    
     
                       TABLE 4                                                     
______________________________________                                    
              Example                                                     
              9     10      11       12                                   
______________________________________                                    
Hardness at 20° C.                                                 
                34      19      7      7                                  
Apparent specific                                                         
                0.417   0.302   0.185  0.182                              
gravity                                                                   
Resilience at -10° C., %                                           
                6.3     7.5     10.3   11.3                               
Resilience at  24° C., %                                           
                3.1     3.7     8.6    9.7                                
Resilience at  40° C., %                                           
                4.3     8.3     10.7   13.5                               
Styrene content, %                                                        
                66.6    66.6    66.6   67.7                               
______________________________________                                    
 
    
     
                       TABLE 5                                                     
______________________________________                                    
             Comparative Example                                          
                            7                                             
                            Commer-                                       
             5              cially                                        
             Chloro-                                                      
                    6       available                                     
                                     8                                    
             prene  EVA     rubber   RB                                   
             foam   foam    sponge   sponge                               
______________________________________                                    
Hardness at 20° C.                                                 
               5        34      28     25                                 
Apparent specific                                                         
               0.189    0.013   0.190  0.62                               
gravity                                                                   
Resilience at -10° C., %                                           
               48.9     54.3    63.0   24.8                               
Resilience at  24° C., %                                           
               51.8     67.6    65.6   48.3                               
Resilience at  40° C., %                                           
               62.3     68.0    64.3   51.4                               
______________________________________                                    
 
    
     Comparison of Table 4 with Table 5 reveals that the rubber foams of the present invention exhibit lower resilience over a wider temperature range than the conventional foams. 
     EXAMPLES 13-16 
     Compositions having formulations shown in Table 6 were milled for 30 minutes through a roll mill having a surface temperature of 70° C. into a sheet, placed into a mold cavity of 25 cm by 30 cm by 1.5 cm, heated to 140° C. under a pressure of 90 kg/cm 2  for 25 mintes, and then taken out of the mold. The results are shown in Table 6. 
     Similar measurements were made on samples of commercially available chloroprene rubber foam, natural rubber foam, EVA foam, RB foam, and special urethane foam (trademark, Solbothane). 
     
                                           TABLE 6                                 
__________________________________________________________________________
                 Example         Comparative Example*                     
                 13  14  15  16  9    10  11  12      13                  
__________________________________________________________________________
Composition, parts by weight                                              
High styrene content                                                      
                 80  80  70  70  natural                                  
                                      EVA RB  chloroprene                 
                                                      urethane            
styrene-butadiene copolymer      rubber                                   
                                      foam                                
                                          foam                            
                                              foam    foam                
Styrene content  60% 55% 60% 55% foam                 (Solbothane ®)  
Natural rubber   20  20  30  30                                           
Filler (calcium carbonate)                                                
                 80  80  80  80                                           
Plasticizer (aromatic oil)                                                
                 30  30  30  30                                           
Zinc oxide       5   5   5   5                                            
Stearic acid     3   3   3   3                                            
Carbon           5   5   5   5                                            
Sulfur           2.5 2.5 2.5 2.5                                          
Accelerator*.sup.8                                                        
                 2.0 2.0 2.0 2.0                                          
Foaming agent*.sup.9                                                      
                 5   5   5   5                                            
Antioxidant      1   1   1   1                                            
Properties                                                                
Apparent specific gravity, g/cm.sup.3                                     
                 0.441                                                    
                     0.456                                                
                         0.445                                            
                             0.463                                        
                                 0.464                                    
                                      0.013                               
                                          0.62                            
                                              0.189   1.31                
Hardness at 20° C., Shore A                                        
                 12  13  10  10  9    34  25  5       8                   
Tensile strength at break, kg/cm.sup.2                                    
                 31.4                                                     
                     36.3                                                 
                         45.3                                             
                             47.8                                         
                                 6.1  6.9 31.0                            
                                              --      1.22                
Elongation at break, %                                                    
                 403 445 473 513 240  90  220 --      610                 
100% modulus     8.5 7.4 6.8 6.3 3.07 --  21.5                            
                                              --      0.23                
300% modulus     21.1                                                     
                     18.6                                                 
                         17.3                                             
                             16.8                                         
                                 --   --  --  --      0.34                
Permanent compressive set**, %                                            
                 36.3                                                     
                     30.3                                                 
                         23.5                                             
                             19.6                                         
                                 0.67 --  --  --      68                  
Resilience at -10° C., %                                           
                 24.5                                                     
                     18.5                                                 
                         27.0                                             
                             14.6                                         
                                 56.0 54.3                                
                                          24.8                            
                                              48.9    15.4                
Resilience at  20° C., %                                           
                 4.0 5.0 4.5 7.3 58.0 67.6                                
                                          48.3                            
                                              51.8    20.0                
Resilience at  50° C., %                                           
                 18.5                                                     
                     24.0                                                 
                         20.0                                             
                             26.3                                         
                                 61.0 68.0                                
                                          51.4                            
                                              62.3    39.5                
__________________________________________________________________________
 *all commercially available foams                                        
 *.sup.8,  *.sup.9 see Table 3                                            
 **test conditions: 70° C., 48 hours, 75% compression.             
 
    
     As seen from Table 6, the rubber foams of the present invention exhibit lower resilience over a wider temperature range than the conventional rubber foams. 
     Samples of Example 13 and Comparative Examples 9, 10 and 13 were further determined for shock-absorbing performance. Using a drop-weight impact tester under conditions of a drop weight of 5 kg, a drop height of 10 cm, and a drop weight-sample contact area of 9 cm 2 , the maximum acceleration was measured. The results are shown in Table 7, which proves that the low-resilience rubber foams of the invention have improved shock absorbing properties. 
     
                       TABLE 7                                                     
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              Example                                                     
                     Comparative Example                                  
              13     9       10      13                                   
______________________________________                                    
Maximum acceleration, G                                                   
                 39      190     180    68                                
Thickness, mm   5.0      4.0     4.9   5.0                                
Thickness-reduced maximum                                                 
                195      760     882   340                                
acceleration, G-mm                                                        
______________________________________                                    
 
    
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.