Process for producing sulfur-curable acrylic rubbers

- Nippon Zeon Co. Ltd.

A process for producing a sulfur-curable acrylic rubber is provided which comprises radical-copolymerizing (1) an alkyl acrylate with (2) an alkenyl acrylate or methacrylate, or radical-copolymerizing (1) an alkyl acrylate, (2) an alkenyl acrylate or methacrylate, (3) an alkoxyalkyl acrylate and optionally (4) acrylonitrile. These acrylic rubbers can be cured at a rate as fast as those for curing diene-type rubbers, and the cured products exhibit superior properties without long heat-treatment after curing.

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Description

This invention relates to a process for producing an acrylic rubber which can be cured with sulfur.

Acrylic rubber is an elastomeric copolymer composed mainly of an acrylate unit, and is known to have superior heat resistance and oil resistance. Since it does not have a double bond at the main chain of the rubber molecules, a monomeric component having an active group capable of becoming a crosslinking site is usually copolymerized with it.

Monomers previously used as the monomeric component for providing crosslinking sites include, for example, halogen-containing monomers such as 2-chloroethyl vinyl ether, vinyl benzyl chloride, vinyl chloroacetate, allyl chloroacetate, and 5-chloroacetoxymethyl-2-norbornene, and epoxy monomers such as allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate.

Because these acrylic rubbers cannot be cured with ordinary sulfur (or sulfur-containing organic compound), vulcanization accelerator systems, amines, ammonium salts, metal soap/sulfur, etc. are generally used as a vulcanizer. Curing with these vulcanizers, however, has the defect that the rate of cure is slow, and to obtain vulcanizates of good properties, the cured products must be heat-treated for a very long period of time after curing.

Furthermore, when the amines or ammonium salts are used as the vulcanizer, unpleasantly odoriferous gases are generated in places where rubber products are made, thus causing a sanitary hazard. This also causes the defect that the cured products cannot be used in medical and foodstuff applications.

The use of the halogen-containing monomers as the crosslinking monomeric component may cause the corrosion of the mold at the time of curing, or the corrosion of metals with which the vulcanized products will make contact.

In an attempt to remove these defects, a method was suggested which involves copolymerization of an acrylate ester with such a crosslinking monomeric component as dicyclopentadiene, methylcyclopentadiene, ethylidene norbornene, vinylidene norbornene, butadiene, isoprene, allyl acrylate, 2-butenyl acrylate, methallyl acrylate, or triallyl isocyanurate to produce an acrylic rubber. The acrylic rubbers obtained by such a method are not entirely satisfactory for practical application because of one or more disadvantages. For example, the rate of cure is extremely slow, or the properties of the cured product are far from meeting the requirements of practical application. Or although the aforesaid problems are solved to some extent, the heat resistance and compression set of the products are still inferior to acrylic rubbers obtained by using the halogen-containing monomers or epoxy monomers as the crosslinking monomeric component.

It is an object of this invention therefore to provide sulfur-curable acrylic rubbers having superior compression set, which permit an improved rate of cure as fast as that for curing natural rubbers and diene-type synthetic rubbers and do not require long heat-treatment after curing, without sacrificing the superior heat resistance, oil resistance, weatherability and ozone resistance of conventional acrylic rubbers.

We have now found that this object can be achieved by using a certain alkenyl acrylate and/or an alkenyl methacrylate as a crosslinking component.

Thus, according to this invention, there is provided a process for producing a sulfur-curable acrylic rubber, which comprises copolymerizing in the presence of a radical initiator (1) 90 to 99.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms and (2) 0.5 to 10% by weight, preferably 2 to 6% by weight, of at least one monomer of the general formula ##STR1## wherein R.sub.1 represents a hydrogen atom or a methyl group, and R.sub.2 and R.sub.3, independently from each other, represent an alkyl group having 1 to 3 carbon atoms.

We have also found that when a part of the alkyl acrylate (1) is replaced by another monomer in the aforesaid process for producing acrylic rubber, there can be obtained an acrylic rubber which has a well balanced combination of cold resistance and oil resistance in addition to the aforesaid favorable properties.

Thus, according to another aspect, there is provided a process for producing a sulfur-curable acrylic rubber, which comprises copolymerizing in the presence of a radical initiator (1) 30 to 89.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms, (2) 0.5 to 10% by weight, preferably 2 to 6% by weight, of at least one monomer of the general formula ##STR2## wherein R.sub.1 represents a hydrogen atom or a methyl group, and R.sub.2 and R.sub.3, independently from each other, represent an alkyl group containing 1 to 3 carbon atoms,

(3) 10 to 60% by weight, preferably 20 to 50% by weight, of at least one alkoxyalkyl acrylate with the alkoxy group containing 1 to 4 and the alkylene group containing 1 to 4 carbon atoms, and (4) 0 to 30% by weight, preferably 2 to 20% by weight, of acrylonitrile.

Examples of the alkyl acrylate (1) are methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, amyl acrylate, hexyl acrylate, cyclohexyl acrylate, and octyl acrylate. Those in which the alkyl group contains 2 to 4 carbon atoms are preferred; for example, they are ethyl acrylate, propyl acrylate and butyl acrylate.

Examples of the alkenyl acrylate and alkenyl methacrylate (2) of general formula (I) include 3-methyl-2-butenyl acrylate, 3-methyl-2-pentyl acrylate, and 3-methyl-2-hexenyl acrylate, and the corresponding methacrylates.

Acrylic rubbers made by the copolymerization of allyl acrylate or 2-butenyl acrylate with alkyl acrylates are known (U.S. Pat. No. 3,476,722). These acrylic rubbers, however, require long heat-treatment after curing as is the case with the conventional acrylic rubbers using the halogen-containing monomers or epoxy monomers as the crosslinking monomeric component, and the properties of the vulcanizates are not satisfactory for practical purposes. Moreover, acrylic rubbers obtained by the ternary copolymerization of allyl methacrylate, an alkyl acrylate and acrylonitrile are also known (Japanese Patent Publication No. 7893/72). But these acrylic rubbers have poor mechanical strength, and cannot find practical application.

When in accordance with this invention, at least one monomer of general formula (I) (i.e., an alkenyl acrylate or alkenyl methacrylate) is used as the crosslinking monomer, these defects can be markedly remedied. If the amount of the monomer (2) is less than 0.5% by weight, the rate of cure is extremely low, and vulcanizates having satisfactory properties for practical application cannot be obtained. If the amount of the monomer (2) exceeds 10% by weight, the cured product has very high hardness, and a greatly reduced elongation, and therefore, cannot be used satisfactorily as a rubber product.

Examples of the alkoxyalkyl acrylate (3) include methoxymethyl acrylate, methoxyethyl acrylate, ethoxyethyl acrylate, and butoxyethyl acrylate.

The acrylonitrile (4) is used in the aforesaid amounts when it is necessary to adjust the strength and oil resistance of the acrylic rubber of this invention.

The process of this invention can be easily performed by polymerizing a mixture of the monomers (1) and (2) or a mixture of the monomers (1), (2) and (3) and optionally (4) in the presence of a radical initiator by a known polymerization method (e.g., in emulsion, suspension, solution, bulk, etc.). The polymerization is performed batchwise, or while adding at least one component continuously and/or intermittently during the reaction. The polymerization temperature is preferably from -10.degree. C. to +100.degree. C., more preferably from 2.degree. to 80.degree. C.

The resulting acrylic rubber can be easily cured with sulfur vulcanization systems generally used for natural rubbers or diene-type synthetic rubbers, and this curing treatment does not require the very long heat-treatment after curing which is essential in the curing of ordinary acrylic rubbers having a halogen containing monomer or epoxy monomer as crosslinking sites. The cured products have superior heat resistance, weatherability and ozone resistance, and in some cases, a well balanced combination of cold resistance and oil resistance. Furthermore, the acrylic rubbers of this invention have markedly improved heat resistance and compression set over the known sulfur-curable acrylic rubbers.

In addition to the vulcanization system, various compounding agents such as reinforcing agents, fillers, plasticizers and antioxidants may be added to the rubbers of this invention as required.

The cured products of the acrylic rubbers of this invention are useful in many applications which require heat resistance, oil resistance, weatherability and ozone resistance. They include, for example, various rubber products such as gaskets, hoses, conveyor belts, packings, oil seals and valve seats.

The present invention is more specifically illustrated below with reference to Examples and the accompanying drawings.

In the drawings,

FIG. 1 shows cure curves of acrylic rubbers obtained in Runs Nos. 3, 9 and 10 in Example 1 which were measured by means of an oscillating disc rheometer (a product of Toyo Seiki K.K.); and

FIG. 2 show cure curves of acrylic rubbers obtained in Runs Nos. 1, 7 and 8 of Example 2 measured in the same way as described above. In these graphic representations, the abscissa represents the cure time (minutes), and the ordinates, the torque (kg.cm). The solid line refers to the example within the scope of this invention, and the broken lines, to comparisons.

EXAMPLE 1

A series of copolymers were produced by using the monomeric mixtures shown in Table 1. The method of polymerization was as follows:

A 2-liter separable flask equipped with a thermometer, a stirrer, a nitrogen introducing tube and an evacuation device was charged with a mixture of the following formulation.

  ______________________________________                                    

     Water                    1,000 g                                          

     Sodium dodecylbenzenesulfonate                                            

                              20 g                                             

     Sodium naphthalenesulfonate                                               

                              10 g                                             

     Sodium sulfate           3 g                                              

     Tetrasodium ethylenediamine-                                              

     tetraacetate             0.2 g                                            

     Ferrous sulfate          0.005 g                                          

     Monomeric mixture (Table 1)                                               

                              1000 g                                           

     ______________________________________                                    

The pH of the mixture was adjusted to 7, and with stirring, the temperature of the inside of the flask was maintained at 5.degree. C. Furthermore, the inside of the flask was deoxygenated fully by using the nitrogen introducing tube and the evacuation device. Then, the following compounds were added, and the reaction was started.

Na.sub.2 S.sub.2 O.sub.4 :0.2 g

Sodium formaldehyde sulfoxylate: 0.2 g

p-Menthane hydroperoxide: 0.1 g

The polymerization was terminated in about 16 hours. The polymerization conversion was within the range of 95 to 99% in each run. After the polymerization, the reaction mixture was salted out, washed with water, and dried under vacuum in a dryer to form the desired acrylic rubber.

                TABLE 1                                                     

     ______________________________________                                    

     Monomers     Run No                                                       

     (parts by    Invention     Comparison                                     

     weight)      1     2     3   4   5   6   7   8   9   10                   

     ______________________________________                                    

     Ethyl acrylate                                                            

                  99    97    95  97  32  97  97  97  98  95                   

     Butyl acrylate                                                            

                  --    --    --  --  65  --  --  --  --  --                   

     3-Methyl-2-butenyl                                                        

     acrylate      1     3     5  --   3  --  --  --  --  --                   

     Allyl acrylate                                                            

                  --    --    --  --  --   3  --  --  --  --                   

     2-Butenyl acrylate                                                        

                  --    --    --  --  --  --   3  --  --  --                   

     Methallyl acrylate                                                        

                  --    --    --  --  --  --  --   3  --  --                   

     Glycidyl                                                                  

     methacrylate --    --    --  --  --  --  --  --   2  --                   

     2-Chloroethylvinyl                                                        

     ether        --    --    --  --  --  --  --  --  --   5                   

     ______________________________________                                    

Using the 10 acrylic rubbers obtained, compounds were prepared by means of an open roll in accordance with the recipes shown in Table 2. The compounds were each press-cured at 160.degree. C. for 20 minutes, and then heat-treated in a Geer oven at 150.degree. C. for 4 hours and 16 hours, respectively.

                TABLE 2                                                     

     ______________________________________                                    

     Recipe          Run No.                                                   

     (parts by weight)                                                         

                     1-8       9       10                                      

     ______________________________________                                    

     Acrylic rubber  100       100     100                                     

     Stearic acid    1         1       1                                       

     HAF-LS carbon black                                                       

                     60        60      60                                      

     Zinc oxide No. 1                                                          

                     3         --      --                                      

     Red lead        --        --      5                                       

     2-Mercaptobenzimida-                                                      

     zole            --        --      1.5                                     

     Ammonium benzoate                                                         

                     --        1       --                                      

     Tetramethylthiuram                                                        

     disulfide       2         --      --                                      

     4,4'-Dithiomorpholine                                                     

                     2         --      --                                      

     N--cyclohexyl-2-benzo-                                                    

     thiazyl sulfenamide                                                       

                     1         --      --                                      

     ______________________________________                                    

The properties of the cured products were measured in accordance with JIS K-6301, and the results are shown in Table 3.

The cure behaviors of a compound containing the acrylic rubber of this invention (Run No. 3) and compounds containing conventional acrylic rubbers including an epoxy monomer and a halogen-containing monomer as the crosslinking monomer (Runs Nos. 9 and 10) were measured by an oscillating disc rheometer. The relation between the curing time and the torque were plotted in FIG. 1.

                                    TABLE 3-1                               

     __________________________________________________________________________

                   Run No.                                                     

                   Invention      Comparison                                   

     Test items    1  2  3  4  5  6  7  8  9  10                               

     __________________________________________________________________________

     Properties after curing                                                   

     at 160.degree. C. for 20 minutes                                          

     Tensile strength (kg/cm.sup.2)                                            

                   121                                                         

                      153                                                      

                         152                                                   

                            111                                                

                               140                                             

                                  64 73 70 106                                 

                                              98                               

     100% Tensile stress (kg/cm.sup.2)                                         

                   11 36 49 18 31 11 15 13 40 19                               

     Elongation (%)                                                            

                   710                                                         

                      330                                                      

                         190                                                   

                            510                                                

                               340                                             

                                  320                                          

                                     300                                       

                                        300                                    

                                           340                                 

                                              490                              

     Hardness (Shore A)                                                        

                   50 67 69 54 65 48 52 50 63 58                               

     Compression set (%)(*)                                                    

                   69 54 55 76 68 84 80 83 80 89                               

     __________________________________________________________________________

      (*): Compression ratio 25%, 150.degree. C. .times. 70 hours              

                                    TABLE 3-2                               

     __________________________________________________________________________

                   Run No.                                                     

                   Invention      Comparison                                   

     Test items    1  2  3  4  5  6  7  8  9  10                               

     __________________________________________________________________________

     Properties after curing at 160.degree. C. for 20 minutes and then         

     heat-treatment at 150.degree. C. for 4 hours                              

     __________________________________________________________________________

     Tensile strength (kg/cm.sup.2)                                            

                   134                                                         

                      149                                                      

                         149                                                   

                            120                                                

                               139                                             

                                  67 81 73 113                                 

                                              119                              

     100% Tensile stress (kg/cm.sup.2)                                         

                   14 36 49 19 36 15 23 20 53 31                               

     Elongation (%)                                                            

                   600                                                         

                      320                                                      

                         190                                                   

                            470                                                

                               310                                             

                                  270                                          

                                     280                                       

                                        270                                    

                                           290                                 

                                              350                              

     Hardness (Shore A)                                                        

                   54 67 69 57 67 54 58 55 66 62                               

     Compression set (%)(*)                                                    

                   30 27 22 38 34 63 56 61 56 77                               

     __________________________________________________________________________

     Properties after curing at 160.degree. C. for 20 minutes and then         

     heat-treatment at 150.degree. C. for 16 hours                             

     __________________________________________________________________________

     Tensile strength (kg/cm.sup.2)                                            

                   Since the properties reached                                

                                  69 70 68 134                                 

                                              137                              

     100% Tensile stress (kg/cm.sup.2)                                         

                   an equilibrium as a result of                               

                                  21 29 25 69 62                               

     Elongation (%)                                                            

                   the curing at 160.degree. C. for 20                         

                                  230                                          

                                     220                                       

                                        220                                    

                                           210                                 

                                              220                              

     Hardness (Shore A)                                                        

                   minutes and heat treatment at                               

                                  59 61 60 69 69                               

     Compression set (%)(*)                                                    

                   150.degree. C. for 4 hours, the                             

                                  51 44 45 29 59                               

                   testing of the properties                                   

                   after the 16-hours heat-                                    

                   treatment was omitted                                       

     __________________________________________________________________________

                                    TABLE 3-3                               

     __________________________________________________________________________

                  Run No.                                                      

                  Invention           Comparison                               

     Test items   1   2   3   4   5   6   7   8   9   10                       

     __________________________________________________________________________

     Heat aging test (in Geer oven at 175.degree. C. for 170 hours) (**)       

     Percent change in tensile                                                 

     strength (%) -47 -28 -4  -32 -27 -48 -26 -41 +15 -37                      

     Percent change in elonga-                                                 

     tion (%)     +8  .+-.0                                                    

                          -8  -15 -7  -48 -32 -50 -20 -43                      

     Change in hardness (point)                                                

                  +12 +11 +12 +15 +13 +20 +18 +20 +9  +8                       

     Heat aging test in oil (150.degree. C. for 70 hours in ASTM No. 3 oil)    

     (**)                                                                      

     Percent change in volume                                                  

     (%)          +17.0                                                        

                      +15.1                                                    

                          + 13.6                                               

                              +16.2                                            

                                  +26.3                                        

                                      +16.5                                    

                                          +16.0                                

                                              +16.5                            

                                                  +16.7                        

                                                      +17.8                    

     Cold resistance test (low-temperature torsion test in JIS K-6301 (**)     

     T-10 (.degree.C.)                                                         

                  -12.0                                                        

                      -10.0                                                    

                          -9.5                                                 

                              -10.0                                            

                                  -21.0                                        

                                      -10.0                                    

                                          -10.0                                

                                              -10.0                            

                                                  -10.0                        

                                                      -10.0                    

     __________________________________________________________________________

      (**): In Runs Nos. 1 to 8 samples cured at 160.degree. C. for 20 minutes 

      (not heattreated subsequently) were used, and in Runs Nos. 9 and 10,     

      samples cured at 160.degree. C. for 20 minutes and then heattreated at   

      150.degree. C. for 16 hours were used.                                   

It is seen from FIG. 1 that while the conventional acrylic rubbers (Runs Nos. 9 and 10) cure very slowly, the acrylic rubber of this invention (Run No. 3) cures very rapidly after starting of vulcanization and then reaches an equilibrium, thus exhibiting the same vulcanization behavior as a diene-type rubber. Hence, the acrylic rubber of the invention shows satisfactory properties for practical application when cured at 160.degree. C. for 20 minutes, and its compression set is improved by heat-treatment at 150.degree. C. for as short as 4 hours. To obtain vulcanizates having properties equivalent to the acrylic rubber of this invention, the conventional acrylic rubbers must be heat-treated at 150.degree. C. for as long as 16 hours.

Acrylic rubbers including allyl acrylate, 2-butenyl acrylate, and methallyl acrylate (Runs Nos. 6, 7 and 8, respectively) have very poor roll processability, and even when heat-treated in the same way as in the case of the conventional acrylic rubbers, cannot have satisfactory vulcanization properties for practical application.

It is appreciated from the experimental results given above that the acrylic rubber of this invention is an acrylic rubber of very good quality which shows equivalent or better vulcanization properties to or than the conventional acrylic rubbers without performing the long heat-treatment which is essential to the vulcanization of the conventional acrylic rubbers.

EXAMPLE 2

A 2-liter separable flask equipped with a thermometer, a stirrer, a nitrogen introducing tube and an evacuation device was charged with a mixture of recipe (I) below. The pH of the mixture in the flask was adjusted to 7, and with stirring, the temperature of the inside of the flask was maintained at 5.degree. C. The oxygen in the flask was sufficiently removed by repeated deaeration and nitrogen introduction. Then, a mixture of recipe (II) below was added, and the polymerization was started. The polymerization was terminated in about 16 hours. The polymerization conversion was within the range of 95 to 99% in each run. After the polymerization, the product was salted out, washed thoroughly with water, and dried under reduced pressure in a dryer to obtain the desired acrylic rubber.

Polymerization Recipe

Recipe (I)

  ______________________________________                                    

     Water                    1000 g                                           

     Sodium dodecylbenzenesulfonate                                            

                              20 g                                             

     Sodium naphthalenesulfonate                                               

                              10 g                                             

     Sodium sulfate           3 g                                              

     Tetrasodium ethylenediamine-                                              

     tetraacetate             0.2 g                                            

     FeSO.sub.4               0.005 g                                          

     Monomeric mixture (Table 4)                                               

                              1000 g                                           

     ______________________________________                                    

Recipe (II)

  ______________________________________                                    

     Na.sub.2 S.sub.2 O.sub.4                                                  

                           0.2 g                                               

     Sodium formaldehyde sulfoxylate                                           

                           0.2 g                                               

     p-Menthane hydroperoxide                                                  

                           0.1 g                                               

     ______________________________________                                    

                TABLE 4                                                     

     ______________________________________                                    

                 Run No.                                                       

     Monomers      Invention       Comparison                                  

     (parts by weight)                                                         

                   1     2     3   4   5   6   7     8                         

     ______________________________________                                    

     Ethyl acrylate                                                            

                   53    47    37  --  52  52  98    95                        

     Butyl acrylate                                                            

                   25    15    --  47  20  25  --    --                        

     Methoxyethyl acrylate                                                     

                   20    35    60  50  20  20  --    --                        

     Acrylonitrile --    --    --  --   5  --  --    --                        

     3-Methyl-2-butenyl                                                        

     acrylate       3     3     3   3   3  --  --    --                        

     3-Methyl-2-hexenyl                                                        

     acrylate      --    --    --  --  --   3  --    --                        

     Glycidyl methacrylate                                                     

                   --    --    --  --  --  --   2    --                        

     2-Chloroethylvinyl                                                        

     ether         --    --    --  --  --  --  --     5                        

     ______________________________________                                    

Using the 8 acrylic rubbers so obtained, compounds were prepared by means of an open roll under cooling in accordance with the recipes shown in Table 5, and then each press-cured at 160.degree. C. for 20 minutes. Then, the cured products were each heat-treated in a Geer oven at 150.degree. C. for 4 hours, and 16 hours, respectively.

                TABLE 5                                                     

     ______________________________________                                    

                    Run No.                                                    

                    Invention                                                  

                            Comparison                                         

     Recipe (parts by weight)                                                  

                      1-6       7       8                                      

     ______________________________________                                    

     Acrylic rubber   100       100     100                                    

     Stearic acid     1         1       1                                      

     HAF-LS carbon black                                                       

                      60        60      60                                     

     Zinc oxide No. 1 3         --      --                                     

     Red lead         --        --      5                                      

     2-Mercapto benzimidazole                                                  

                      --        --      1.5                                    

     Ammonium benzoate                                                         

                      --        1       --                                     

     Tetramethyl thiuram disulfide                                             

                      2         --      --                                     

     4,4'-Dithiomorpholine                                                     

                      2         --      --                                     

     N--Cyclohexyl-2-benzothiazyl                                              

     sulfenamide      1         --      --                                     

     ______________________________________                                    

The properties of the cured products were measured in accordance with JIS K-6301, and the results are shown in Table 6.

The vulcanization behaviors of a compound containing the acrylic rubber of this invention (Run No. 1) and compounds containing the comparative acrylic rubbers (Runs Nos. 7 and 8) were measured in the same way as in Example 1, and the results were plotted in FIG. 2.

                                    TABLE 6-1                               

     __________________________________________________________________________

                   Run No.                                                     

                   Invention                                                   

     Test items    1   2   3   4   5   6   7   8                               

     __________________________________________________________________________

     Properties after curing                                                   

     at 160.degree. C. for 20 minutes                                          

     Tensile strength (kg/cm.sup.2)                                            

                   122 126 134 98  148 103 106 98                              

     100% Tensile stress (kg/cm.sup.2)                                         

                   35  34  35  38  37  36  40  19                              

     Elongation (%)                                                            

                   290 300 310 240 320 240 340 490                             

     Hardness (Shore A)                                                        

                   66  66  66  67  67  67  63  58                              

     Compression set (%) (*)                                                   

                   63  68  67  62  59  73  80  89                              

     __________________________________________________________________________

      (*):Compression ratio 25%, 150.degree. C. .times. 70 hours               

                                    TABLE 6-2                               

     __________________________________________________________________________

                   Run No.                                                     

                   Invention         Comparison                                

     Test items    1  2  3  4  5  6  7  8                                      

     __________________________________________________________________________

     Properties after curing at 160.degree. C. for 20 minutes and then         

     heat-treatment at 150.degree. C. for 4 hours                              

     __________________________________________________________________________

     Tensile strength (kg/cm.sup.2)                                            

                   119                                                         

                      124                                                      

                         131                                                   

                            103                                                

                               141                                             

                                  107                                          

                                     113                                       

                                        119                                    

     100% Tensile stress (kg/cm.sup.2)                                         

                   35 34 34 39 38 39 53 31                                     

     Elongation (%)                                                            

                   290                                                         

                      300                                                      

                         310                                                   

                            240                                                

                               310                                             

                                  220                                          

                                     290                                       

                                        350                                    

     Hardness (Shore A)                                                        

                   66 66 66 67 67 67 66 62                                     

     Compression set (%) (*)                                                   

                   30 33 31 37 32 39 56 77                                     

     __________________________________________________________________________

     Properties after curing at 160.degree. C. for 20 minutes and then         

     heat-treatment at 150.degree. C. for 16 hours                             

     __________________________________________________________________________

     Tensile strength (kg/cm.sup.2)                                            

                   Since the properties reached an                             

                                     134                                       

                                        137                                    

     100% Tensile stress (kg/cm.sup.2)                                         

                   equilibrium as a result of the                              

                                     69 62                                     

     Elongation (%)                                                            

                   curing at 160.degree. C. for 20 minutes                     

                                     210                                       

                                        220                                    

     Hardness (Shore A)                                                        

                   and heat treatment at 150.degree. C. for                    

                                     69 69                                     

     Compression set (%) (*)                                                   

                   4 hours, the testing of the pro-                            

                                     29 59                                     

                   perties after the 16-hour heat-                             

                   treatment was omitted                                       

     __________________________________________________________________________

                                    TABLE 6-3                               

     __________________________________________________________________________

                    Run No.                                                    

                    Invention               Comparison                         

     Test items     1   2   3   4   5   6   7   8                              

     __________________________________________________________________________

     Heat aging test in oil (150.degree. C. for 70 hours in ASTM No. 3 oil)    

     (**)                                                                      

     __________________________________________________________________________

     Percent change in volume (%)                                              

                    +18.9                                                      

                        +13.8                                                  

                            +8.5                                               

                                +22.4                                          

                                    +14.2                                      

                                        +19.7                                  

                                            +16.7                              

                                                +17.8                          

     __________________________________________________________________________

     Cold resistance test (low-temperature torsion test in JIS 6301)           

     __________________________________________________________________________

     (**)                                                                      

     T-10 (.degree.C.)                                                         

                    -21 -21.5                                                  

                            -23.5                                              

                                -34.5                                          

                                    -16 -21.5                                  

                                            -10 -10                            

     __________________________________________________________________________

      (**): In Runs Nos. 1 to 8, samples cured at 160.degree. C. for 20 minutes

      (not heattreated subsequently) were used, and in Runs Nos. 9 and 10,     

      samples cured at 160.degree. C. for 20 minutes and then heattreated at   

      150.degree. C. for 16 hours were used.                                   

It is seen from FIG. 2 that while the conventional acrylic rubbers cure slowly, the acrylic rubber of this invention exhibits a vulcanization behavior equivalent to a diene-type rubber. Accordingly, as shown in Table 6, while the conventional acrylic rubbers require heat-treatment at 150.degree. C. for as long as 16 hours after the curing, the acrylic rubber of this invention affords cured products having equivalent characteristics to the conventional acrylic rubbers by heat-treatment at 150.degree. C. for as short as 4 hours. Moreover, the acrylic rubbers of this invention have markedly improved heat resistance and compression set over the conventional sulfur-curable acrylic rubbers, and exhibit a well balanced combination of cold resistance and oil resistance.

According to this invention, therefore, the slow cure rate of the conventional acrylic rubbers can be increased to a level equivalent to that of diene-type rubbers without impairing the good heat resistance, oil resistance, weatherability, ozone resistance and cold resistance of the conventional acrylic rubbers, and acrylic rubbers having a low compression set and a well balanced combination of cold resistance and oil resistance can be obtained.

Claims

1. A process for producing a sulfur-curable acrylic rubber, which comprises copolymerizing in the presence of a radical initiator

(1) 90 to 99.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms, and
(2) 0.5 to 10% by weight of at least one monomer of the general formula ##STR3## wherein R.sub.1 represents a hydrogen atom or a methyl group, and R.sub.2 and R.sub.3, independently from each other, represent an alkyl group containing 1 to 3 carbon atoms.

2. A process for producing a sulfur-curable acrylic rubber, which comprises copolymerization in the presence of a radical initiator

(1) 30 to 89.5% by weight of at least one alkyl acrylate with the alkyl group containing 1 to 8 carbon atoms,
(2) 0.5 to 10% by weight of at least one monomer of the general formula ##STR4## wherein R.sub.1 represents a hydrogen atom or a methyl group, and R.sub.2 and R.sub.3, independently from each other, represent an alkyl group containing 1 to 3 carbon atoms,
(3) 10 to 60% by weight of at least one alkoxyalkyl acrylate with the alkoxy group containing 1 to 4 carbon atoms and the alkylene group containing 1 to 4 carbon atoms, and
(4) 0 to 30% by weight of acrylonitrile.

3. The process of claim 1 or 2 wherein the monomer (2) represented by the general formula is 3-methyl-2-butenyl acrylate.

4. The process of claim 1 or 2 wherein the monomer (2) represented by the general formula is 3-methyl-2-butenyl methacrylate.

5. The process of claim 1 or 2 wherein the copolymerization is carried out in emulsion.

6. The process of claim 1 or claim 2 wherein the alkyl group of the alkyl acrylate contains 2 to 4 carbon atoms.

7. The process of claim 1 or claim 2 wherein the monomer (2) is present in an amount of from about 2% to about 6% by weight.

8. The process of claim 2 wherein the alkoxyacrylate is present in an amount of from about 20% to about 50% by weight.

9. The process of claim 2 wherein acrylonitrile is present in an amount of from about 2% to about 20% by weight.

10. The process of claim 1 or claim 2 wherein the radical initiator is p-menthane hydroperoxide.

Referenced Cited
U.S. Patent Documents
3201373 August 1965 Kaizerman
3476722 November 1969 Schlatzer
3622547 November 1971 Ermidis
4133793 January 9, 1979 Lewis et al.
Patent History
Patent number: RE31322
Type: Grant
Filed: Sep 23, 1981
Date of Patent: Jul 26, 1983
Assignee: Nippon Zeon Co. Ltd. (Tokyo)
Inventors: Tetsu Ohishi (Tokuyama), Kohichi Handa (Miura), Haruo Ueno (Tokyo)
Primary Examiner: Harry Wong, Jr.
Law Firm: Sherman & Shalloway
Application Number: 6/304,788