Rubber composition
FIELD OF THE INVENTION 
     The present invention relates to a rubber composition. More particularly, it relates to a rubber composition which comprises (A) an ethylene copolymer rubber containing at least one compound selected from the group consisting of acrylic esters and methacrylic esters and ethylene as constituting units and (B) a hydrogenated unsaturated nitrile-conjugated diene copolymer rubber. Vulcanized rubber compositions obtained by vulcanization of the rubber compositions of the present invention are excellent in balancing in low-temperature resistance, oil resistance, heat resistance, ozone resistance, and strength properties and hence, they can be widely used in the fields in which these characteristics are utilized. Some examples are various sealing materials such as gaskets, O-rings, packings and oil seals, various hoses, belts and rolls. 
     BACKGROUND OF THE INVENTION 
     Recently, various apparatuses used in various industrial fields such as automobiles, appliances and machines have been required to have high performances and functions and have been used under increasingly severe conditions. This tendency is especially conspicuous in automobile industries which are largest users for rubbers. For example, the atmosphere in engine rooms tends to be higher in temperature owing to countermeasures against regulation of off-gas or improvement of engines aiming at enhancement of performance such as increase of power output, and rubbers to be employed for such uses are required to be excellent not only in heat resistance, ozone resistance and oil resistance, but also in flexibility at low temperatures, namely, to be well balanced in properties. 
     Ethylene copolymer rubbers comprising ethylene and at least one membered selected from the group consisting of acrylic esters and methacrylic esters as constituting units are excellent in heat resistance, low-temperature resistance and strength characteristics, but are inferior in oil resistance. In order to solve this problem, an attempt has been made to increase contents of acrylic esters and/or methacrylic esters. However, this method has the problems that molecular weight of ethylene copolymer rubbers decreases and elasticity, which is a characteristic of rubbers, is not sufficiently exhibited and besides, improvement of oil resistance is also insufficient. That is, there have not yet been found rubbers or rubber compositions which are well balanced in properties such as strength characteristics, oil resistance, low-temperature resistance, heat resistance and ozone resistance with maintaining the characteristics of rubbers. 
     SUMMARY OF THE INVENTION 
     The present invention provides a rubber composition which comprises the following components (A) and (B): 
     Component (A) an ethylene copolymer rubber containing ethylene and at least one selected from the group consisting of acrylic esters and methacrylic esters as constituting units, and 
     Component (B): a hydrogenated unsaturated nitrile-conjugated diene copolymer rubber. 
     The rubber composition of the present invention can be used finally as a vulcanized rubber composition by vulcanization thereof. 
     Accordingly, the present invention provides a composition from which a vulcanized rubber composition can be prepared, which vulcanized rubber composition is well balanced in such properties as strength characteristics, oil resistance, low-temperature resistance, heat resistance and ozone resistance while retaining the desirable properties characteristic of rubbers. 
     The present invention will be explained in detail. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Component (A) of the present invention is an ethylene copolymer rubber containing ethylene and at least one membered selected from the group consisting of acrylic esters and methacrylic esters as constituting units. 
     The acrylic esters and methacrylic esters used here are usually esters comprising alcohols of 1-8 carbon atoms. Examples of these esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate. These may be used singly or in combination of two or more. 
     Molar ratio of at least one compound selected from the group consisting of acrylic esters and methacrylic esters (hereinafter sometimes reference to as &#34;(meth)acrylic esters&#34;) and ethylene is usually (50-15):(50-85), preferably (42-20):(58-80). If the proportion of the (meth)acrylic ester is too high, embrittlement point of the copolymer is high and it sometimes becomes difficult to use the copolymer as a rubber having elasticity at low temperatures. On the other hand, if the proportion is too low, crystallinity of the copolymer increases and sufficient elasticity may not be obtained. 
     The ethylene copolymer rubber (A) of the present invention can contain a cross-linkable third component in addition to ethylene and (meth) acrylic esters as constituting units. The cross-linkable third component includes, for example, epoxy group-containing monomers and carboxyl group-containing monomers. 
     The epoxy group-containing monomers include, for example, unsaturated glycidyl esters such as glycidyl acrylate, glycidyl methacrylate, diglycidyl itaconate, triglycidyl butenetricarboxylate, and glycidyl p-styrenecaboxylate. 
     The carboxyl group-containing monomers include, for example, half ester compounds of acrylic acid, methacrylic acid and maleic acid and unsaturated acid anhydrides such as maleic anhydride. 
     Content of the crosslinkable third component in the ethylene copolymer rubber (A) is usually 0.05-5 mol %, preferably 0.1-3 mol % based on the total amount of ethylene and (meth)acrylic ester. If the content of the third component is too high, scorching resistance of the resulting vulcanized rubber composition may be unsatisfactory and if it is too low, sometimes sufficient crosslinking effect cannot be obtained. 
     The ethylene copolymer rubber (A) of the present invention may contain, in addition to the above-mentioned monomers, other monomers copolymerizable therewith as constituting units. Such copolymerizable monomers include, for example, styrene and derivatives thereof, vinyl acetate, and halogenated olefins such as tetrafluoroethylene and hexafluoropropylene. 
     The ethylene copolymer rubber (A) is produced by known processes. For example, it can be produced by free-radical-initiated bulk polymerization, emulsion polymerization or solution polymerization. As a representative polymerization process, mention may be made of a process described in JP-B-46-45085, namely, by carrying out the polymerization under the conditions of a pressure of 500 kg/cm 2  or higher and a temperature in the range of 40° to 300° C. in the presence of a free-radical forming polymerization initiator. 
     As the ethylene copolymer rubber (A), there may be used copolymers having a melt index of normally 0.5-500 g/10 min, preferably 0.5-50 g/10 min at 190° C. specified in JIS K6791. If the melt index is too high, sometimes elasticity of the ethylene copolymer rubber is not sufficient and if it is too low, the rubber is sometimes inferior in processability. 
     Component (B) of the present invention is a hydrogenated unsaturated nitrile-conjugated diene copolymer rubber. 
     The unsaturated nitrile includes, for example, acrylonitrile and methacrylonitrile. 
     The conjugated diene includes, for example, 1,3-butadiene, isoprene, and 1,3-pentadiene. These may be used singly or in combination of two or more. 
     As examples of component (B), optional combinations of the unsaturated nitrile and the conjugated diene can be employed and besides, a part of the conjugated dienes can be replaced with unsaturated carboxylic acid esters such as methyl esters, butyl esters, and 2-ethylhexyl esters of acrylic acid, methacrylic acid, fumaric acid, and itaconic acid. Typical examples of component (B) are hydrogenated acrylonitrile-butadiene copolymer rubber, hydrogenated acrylonitrile-isoprene copolymer rubber, hydrogenated acrylonitrile-butadiene-isoprene copolymer rubber, hydrogenated acrylonitrile-butadiene-methyl acrylate copolymer rubber, and hydrogenated acrylonitrile-butadiene-butyl acrylate copolymer rubber. 
     Molar ratio of the unsaturated nitrile and the conjugated diene in the hydrogenated unsaturated nitrile-conjugated diene copolymer rubber (B) is usually (25-60):(75-40), preferably (35-55):(65-45). If proportion of the unsaturated nitrile is too high, the resulting vulcanized rubber composition is sometimes inferior in low-temperature resistance. If it is too low, the resulting vulcanized rubber composition is sometimes inferior in oil resistance. 
     The degree of hydrogenation of the conjugated diene unit portion of the hydrogenated unsaturated nitrile-conjugated diene copolymer rubber (B) is usually 50-98 mol %, preferably 70-98 mol %. With increase in the hydrogenation degree, heat resistance and ozone resistance of the resulting vulcanized rubber composition are improved and the improvements become the highest when hydrogenation degree is 100 mol %. However, in general the copolymer rubbers which are somewhat lower than 100 mol % in the hydrogenation degree are used taking into consideration the desired level of heat resistance and ozone resistance and the cost. 
     The hydrogenated unsaturated nitrile-conjugated diene copolymer rubber (B) can be obtained by hydrogenating the conjugated diene unit portion in unsaturated nitrile-conjugated diene copolymer rubbers produced by emulsion polymerization or solution polymerization by the processes disclosed in, for example, JP-B-45-39275 and JP-A-52-32095. 
     Proportion of ethylene copolymer rubber (A) and hydrogenated unsaturated nitrile-conjugated diene copolymer rubber (B) is not critical, but is usually (95-5):(5-95), preferably (70-30):(30-70) in weight ratio. If proportion of the ethylene copolymer rubber (A) is too high, the resulting vulcanized rubber composition is sometimes inferior in oil resistance. If it is too low, the resulting vulcanized rubber composition is sometimes inferior in ozone resistance and heat resistance. 
     The rubber composition of the present invention is used in the form of a vulcanizable rubber composition containing vulcanizing agent (C). As the vulcanizing agent (C), there may be used organic peroxides used for crosslinking of rubbers. Examples thereof are di-t-butyl peroxide, t-butylcumyl peroxide, dicumyl peroxide, α,α-bis(t-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-bis(t-butylperoxy) valerate, 2,2-bis(t-butylperoxy)butane, and 2,2-bis(t-butylperoxy)octane. 
     Content of vulcanizing agent (C) is preferably in the range of 1 to 10 parts by weight, more preferably in the range of 2 to 6 parts by weight per totally 100 parts by weight of ethylene copolymer rubber (A) and hydrogenated unsaturated nitrile-conjugated diene copolymer rubber (B). If content of vulcanizing agent (C) is too high, problems such as blowing occur in vulcanization molding and if it is too low, crosslinking density is low and mechanical strength of vulcanized rubber composition is sometimes insufficient. 
     The rubber composition of the present invention is mixed with the above-mentioned vulcanizing agent and reinforcing agents, fillers, plasticizers, aging inhibitors, stabilizers, processing aids, etc. which are normally used in rubber industries by usual kneading machines such as roll and Banbury mixer, thereby to prepare a vulcanizable rubber composition. The composition is molded into various shapes depending on its use. Vulcanization is carried out usually at about 120° C. or higher, preferably at about 150° to 220° C. for about 1 to 30 minutes. 
    
    
     The present invention will be explained in detail by the following examples. 
     EXAMPLES 1-3 AND COMPARATIVE EXAMPLES 1-2 
     Ethylene copolymer rubber and hydrogenated acrylonitrile-butadiene copolymer rubber (hereinafter sometimes referred to as &#34;hydrogenated NBR&#34;) as hydrogenated unsaturated nitrile-conjugated diene copolymer rubber which are shown in the column of Rubber component in Table 1, 40 parts by weight of FEF black, 1 part by weight of stearic acid, 0.5 part by weight of ARMEEN 18D (a processing aid manufactured by Lion-Akzo Co.), 2 parts by weight of NAUGUARD 44 (an aging inhibitor manufactured by Uniroyal Chemical Co.), 3.5 parts by weight of 98% dicumyl peroxide (an organic peroxide manufactured by Sanken Kako Co.), and 1.0 part by weight of TAIC (a processing sid manufactured by Nippon Kasei Co.) were kneaded by a 6 inch open roll at 50° to 60° C. for 15 to 20 minutes to obtain a blend. This was press vulcanized at 170° C. for 20 minutes. Properties of the resulting vulcanized rubber compositions were measured in accordance with JIS K6301. The results are shown in Table 1. 
     EXAMPLES 4-7 AND COMPARATIVE EXAMPLES 3-5 
     Example 1 was repeated except that hydrogenated NBR differing in acrylonitrile/butadiene contents and hydrogenation degree were used. The conditions and the results are shown in Table 2. 
     EXAMPLES 8-11 AND COMPARATIVE EXAMPLES 6-8 
     Example 1 was repeated except that ethylene copolymer rubbers different in ethylene/methyl acrylate contents from that in Example 1 were used. The conditions and the results are shown in Table 3. 
     EXAMPLES 12-16 AND COMPARATIVE EXAMPLES 9-12 
     Example 1 was repeated except that ethylene copolymer rubbers which comprised ethylene, methyl acrylate and besides other monomers were used and a vulcanizing agent different from that in Example 1 was used in some cases. The conditions and the results are shown in Table 4-1 and Table 4-2. 
     
                       TABLE 1                                                     
______________________________________                                    
                          Comparative                                     
              Example     Example                                         
              1     2      3      1     2                                 
______________________________________                                    
Rubber component                                                          
(part by weight)                                                          
(A)  Ethylene copolymer                                                   
                    70      50   30   100   --                            
     rubber*1                                                             
     Type-1                                                               
(B)  Hydrogenated NBR*2                                                   
                    30      50   70   --    100                           
     Zetpol 2010                                                          
Evaluation of valcanized                                                  
rubber                                                                    
Tensile properties                                                        
Tensile strength                                                          
                151     192    223  141   261                             
Kgf/cm.sup.2                                                              
Elongation %    280     310    350  350   420                             
Hardness JIS-A  62      63     63   55    65                              
Heat resistance*3                                                         
Change in tensile                                                         
                -3      -10    -23  -16   -23                             
strength %                                                                
Change in elongation                                                      
                -46     -65    -71  +26   -74                             
Change in hardness                                                        
                +20     +18    +17  +8    +16                             
Point                                                                     
Oil resistance*4                                                          
Volume swell %  +52     +42    +34  +70   +23                             
Low-temperature resist-                                                   
ance (Gehman torsional                                                    
test)                                                                     
T.sub.2 °C.  -17     -9   - 19 -7    -20                           
T.sub.5 °C.  -28     -25  -27  -26   -25                           
T.sub.10                                                                  
        °C.  -30     -28  -29  -31   -27                           
T.sub.100                                                                 
        °C.  -38     -35  -35  -37   -33                           
Freezing temperature                                                      
                -40     -36    -35  -40   -34                             
°C.                                                                
______________________________________                                    
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
                              Comparative                                 
                  Example     Example                                     
                  4  5  6  7  3  4  5                                     
__________________________________________________________________________
Rubber components                                                         
(part by weight)                                                          
(A)                                                                       
   Ethylene copolymer rubber*1                                            
                  70 30 70 30 100                                         
                                 -- --                                    
   Type-1                                                                 
(B)                                                                       
   Hydrogenated NBR*2                                                     
   Zetpol 1020    30 70 -- -- -- 100                                      
                                    --                                    
   Zetpol 2020    -- -- 30 70 -- -- 100                                   
Evaluation of valcanized                                                  
rubber                                                                    
Tensile properties                                                        
Tensile strength Kgf/cm.sup.2                                             
                  150                                                     
                     218                                                  
                        150                                               
                           215                                            
                              141                                         
                                 275                                      
                                    254                                   
Elongation %      260                                                     
                     290                                                  
                        230                                               
                           290                                            
                              350                                         
                                 360                                      
                                    340                                   
Hardness JIS-A    64 67 63 65 55 69 68                                    
Heat Resistance*5                                                         
Change in tensile strength %                                              
                  -9 -42                                                  
                        -1 -44                                            
                              -16                                         
                                 -56                                      
                                    -57                                   
Change in elongation %                                                    
                  -62                                                     
                     -57                                                  
                        -42                                               
                           -50                                            
                              +26                                         
                                 -83                                      
                                    -82                                   
Change in hardness Point                                                  
                  +23                                                     
                     +17                                                  
                        +20                                               
                           + 16                                           
                              +8 +13                                      
                                    +12                                   
Oil resistance*4                                                          
Volume swell %    +48                                                     
                     +25                                                  
                        +51                                               
                           +33                                            
                              +70                                         
                                 +10                                      
                                    +22                                   
Low-temperature resist-                                                   
ance (Gehman torsional test)                                              
T.sub.2                                                                   
      °C.  -11                                                     
                     -10                                                  
                        -17                                               
                           -21                                            
                              -7 -14                                      
                                    -22                                   
T.sub.100                                                                 
      °C.  -35                                                     
                     -26                                                  
                        -37                                               
                           -35                                            
                              -37                                         
                                 -27                                      
                                    -34                                   
Ozone resistance                                                          
(Generation of cracks)*6                                                  
48 hr             No No No No No No B-2                                   
96 hr             No No No No No B-2                                      
                                    B-3                                   
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                                           TABLE 3                                 
__________________________________________________________________________
                              Comparative                                 
                  Example     Example                                     
                  8  9  10 11 6  7  8                                     
__________________________________________________________________________
Rubber components                                                         
(part by weight)                                                          
(A)                                                                       
   Ethylene copolymer rubber*1                                            
   Type-2         70 30 -- -- 100                                         
                                 -- --                                    
   Type-3         -- -- 70 30 -- 100                                      
                                    --                                    
(B)                                                                       
   Hydrogenated NBR*2                                                     
   Zetpol 2020    30 70 30 70 -- -- 100                                   
Evaluation of vulcanized rubber                                           
Tensile properties                                                        
Tensile strength kgf/cm.sup.2                                             
                  152                                                     
                     194                                                  
                        140                                               
                           191                                            
                              149                                         
                                 141                                      
                                    254                                   
Elongation %      230                                                     
                     260                                                  
                        220                                               
                           260                                            
                              330                                         
                                 350                                      
                                    340                                   
Hardness JIS-A    63 66 64 65 56 56 68                                    
Heat resistance*5                                                         
Change in tensile strength %                                              
                  -14                                                     
                     -47                                                  
                        -10                                               
                           -44                                            
                              -21                                         
                                 -24                                      
                                    -57                                   
Change in elongation %                                                    
                  -61                                                     
                     -77                                                  
                        -59                                               
                           -77                                            
                              +21                                         
                                 +20                                      
                                    -82                                   
Change in hardness Point                                                  
                  +20                                                     
                     +14                                                  
                        +21                                               
                           +16                                            
                              +5 +6 +12                                   
Oil resistance*4                                                          
Volume swell %    +58                                                     
                     +37                                                  
                        +47                                               
                           +33                                            
                              +83                                         
                                 +59                                      
                                    +22                                   
Low-temperature resist-                                                   
ance (Gehman torsional test)                                              
T.sub.2                                                                   
      °C.  -18                                                     
                     -22                                                  
                        -16                                               
                           -19                                            
                              -10                                         
                                 +5 -22                                   
T.sub.100                                                                 
      °C.  -38                                                     
                     -35                                                  
                        -36                                               
                           -34                                            
                              -39                                         
                                 -32                                      
                                    -34                                   
Ozone resistance                                                          
(Generation of cracks)*6                                                  
48 hr             No No No No No No B-2                                   
96 hr             No No No No No No B-3                                   
__________________________________________________________________________
 
    
     
                       TABLE 4-1                                                   
______________________________________                                    
              Example                                                     
              12   13      14     15   16                                 
______________________________________                                    
Rubber components                                                         
(part by weight)                                                          
(A)  Ethylene copolymer                                                   
     rubber*1                                                             
     Type-4         70     30    --   --   --                             
     Type-5         --     --    70   30   --                             
     Type-1         --     --    --   --   50                             
(B)  Hydrogenated NBR*2                                                   
     Zetpol 2020    30     70    30   70   50                             
Evaluation of vulcanized                                                  
rubber                                                                    
Tensile properties                                                        
Tensile strength                                                          
                154    210     146  197  110                              
Kgf/cm.sup.2                                                              
Elongation %    240    270     250  270  500                              
Hardness JIS-A  64     65      63   65   58                               
Heat resistance*5                                                         
Change in tensile                                                         
                -5     -46     -9   -43  0                                
strength %                                                                
Change in elongation                                                      
                -42    -65     -64  -78  -84                              
Change in hardness                                                        
                +20    +16     +20  +16  +21                              
Point                                                                     
Oil resistance*4                                                          
Volume swell %  +50    +33     +52  +34  +57                              
Low-temperature resist-                                                   
ance (Gehman torsional                                                    
test)                                                                     
T.sub.2 °C.  -12    -18   -12  -17  -12                            
T.sub.100                                                                 
        °C.  -36    -34   -36  -34  -36                            
Ozone resistance                                                          
(Generation of cracks)*6                                                  
48 hr           No     No      No   No   No                               
96 hr           No     No      No   No   No                               
______________________________________                                    
 
    
     
                       TABLE 4-2                                                   
______________________________________                                    
               Comparative Example                                        
                 9       10      11    12                                 
______________________________________                                    
Rubber components                                                         
(part by weight)                                                          
(A)  Ethylene copolymer                                                   
     rubber*1                                                             
     Type-4          100     --    --    --                               
     Type-5          --      100   --    --                               
     Type-1          --      --    100   --                               
(B)  Hydroganated NBR*2                                                   
     Zetpol 2020     --      --    --    100                              
Evaluation of vulcanized                                                  
rubber                                                                    
Tensile properties                                                        
Tensile strength 142     135     52    254                                
Kgf/cm.sup.2                                                              
Elongation %     300     420     770   340                                
Hardness JIS-A   57      56      47    68                                 
Heat resistance*5                                                         
Change in tensile                                                         
                 -1      -32     -40   -57                                
strength %                                                                
Change in elongation                                                      
                 -57     +17     -22   -82                                
Change in hardness                                                        
                 +15     +2      +6    +12                                
Point                                                                     
Oil resistance*4                                                          
Volume swell %   +63     +69     +100  +22                                
Low-temperature resistance                                                
(Gehman torsional test)                                                   
T.sub.2 °C.   -10     -6    -6    -22                              
T.sub.100                                                                 
        °C.   -37     -35   -35   -34                              
Ozone resistance                                                          
(Generation of cracks)*6                                                  
48 hr            No      No      No    B-2                                
96 hr            No      No      No    B-3                                
______________________________________                                    
 In Comparative Example 11, dit-butyl-peroxy-3,3,5-trimethylcyclohexane wa
 used as an organic peroxide.                                             
 *1: Ethylene copolymer rubbers                                           
 Type-1: Ethylene 68 mol %, methyl acrylate 32 mol %                      
 Type-2: Ethylene 73 mol %, methyl acrylate 27 mol %                      
 Type-3: Ethylene 56 mol %, methyl acrylate 44 mol %                      
 Type-4: Ethylene 67 mol %, methyl acrylate 32 mol %, glycidyl methacrylat
 1 mol %                                                                  
 Type-5: VAMAC G manufactured by DuPont de Nemours, E. I. , Co. [ethylene 
 (41 mol %) - methyl acrylate (54 mol %) - carboxylgroup containing monome
 (5 mol %) copolymer rubber]-                                             
 *2: Hydrogenated NBR                                                     
 Zetpol 2010: manufactured by Nippon Zeon Co., Ltd.;                      
 acrylonitrile:butadiene molar ratio = 37:63; hydrogenation degree: 95 mol
 %                                                                        
 Zetpol 1020: manufactured by Nippon Zeon Co., Ltd.;                      
 acrylonitrile:butadiene molar ratio = 45:55; hydrogenation degree: 90 mol
 %                                                                        
 Zetpol 2020: manufactured by Nippon Zeon Co., Ltd.;                      
 acrylonitrile:butadiene molar ratio = 37:63; hydrogenation degree: 90 mol
 %                                                                        
 *3: Aging test by heating in the air: 175° C. × 7 days      
 *4: JIS No. 3 oil: 150° C. × 70 hours                       
 *5: Aging test by heating in the air: 185° C. × 3 days      
 *6: Static test: Stretching 20 % at 60° C. and 50 pphm O.sub.3 -  
 
    
     The compositions in all of the above Examples were satisfactory in all properties of tensile properties, heat resistance, oil resistance and low-temperature resistance and it can be seen that vulcanized rubber compositions well-balanced in properties were obtained. On the other hand, in Comparative Examples, compositions which did not contain component (A) of the present invention were inferior in heat resistance and those which did not contain component (B) of the present invention were inferior in tensile properties and oil resistance and all of them lacked balance of properties. 
     As explained above, according to the present invention, it has become possible to provide a rubber composition which can be made to a vulcanized rubber composition having well-balanced strength characteristics, oil resistance, low-temperature resistance, heat resistance and ozone resistance with maintaining the characteristics of rubbers.