Primer compositions
BACKGROUND OF THE INVENTION 
     This invention relates to novel primer compositions. More particularly, the invention relates to novel primer compositions useful for forming a strong adhesive bond between the substrate and an overlaying room-temperature-curing silicone rubber composition. 
     In the application and cure of room-temperature-curing silicone rubber compositions to and on substrate, a variety of primers have been used to produce a strong adhesive bond between the cured silicone elastomers and the substrates. The known primers comprise, for example, alkoxy silanes, such as, vinyl trimethoxysilane, 3-glycidoxypropyl trimethoxysilane, 3-methacryloxypropyl trimethoxysilane, and N(2-aminoethoxy)-3-aminopropyl trimethoxysilane, condensates of partial hydrolyzates or cohydrolyzates of these silanes, titanic acid esters, metal salts of fatty acids, or peroxysilanes. Those primers can, advantageously, serve to make room-temperature-curing silicone rubber adhere to an iron or aluminum surface but disadvantageously, they do not so work when the silicone rubber is cured by the crosslinking mechanism of dehydroxylamination reaction. Further the known primers are disadvantaged by the facts that they show little effectiveness to glass, mortar and asbestos surfaces and that the cured silicone rubber adhesively bonded to the substrate of iron or aluminum with these primers tends to readily peel off when affected by water. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide new primer compositions free of the above-mentioned disadvantages existent in the conventional primers. 
     It is another object of the present invention to provide a method for adhesively bonding a room-temperature-curing silicone rubber elastomer to a substrate by applying the prime compositions to the surface of the substrate. 
     The primer compositions of the invention consist essentially of 2 to 98 parts by weight of a mercapto-containing organosilane or organopolysiloxane represented by the average unit formula ##EQU1## WHERE R 1  is an alkylene group having 1 to 4 carbon atoms, R 2  is a substituted or unsubstituted monovalent hydrocarbon group, X is a hydroxy or hydrolyzable atom or group and a, b, and c have values as expressed by 0&lt;a≦1, 0≦b≦1 and 0&lt;c≦3, respectively, with the proviso that a + b + c has the value as expressed by 0&lt;(a + b + c)≦4 and 98 to 2 parts by weight of a titanic acid ester. 
     The primer compositions of the present invention can serve to make a room-temperature-curing silicone rubber elastomer strongly bond to various surfaces contacting it, the surfaces being made of metals including aluminum, copper, stainless steel and brass, plastics, including epoxy resins, vinyl chloride resins and polyester resins, or inorganic substances including glass, mortar and asbestos, and the bonds thus formed being resistant to water and sea water. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To describe the invention in further detail, the organosilanes or organosiloxanes as the first component in the primer composition are expressed by the above-mentioned formula, in which the alkylene groups having 1 to 4 carbon atoms denoted by R 1  are exemplified by methylene, ethylene, propylene and butylene groups; the substituted or unsubstituted monovalent hydrocarbon groups denoted by R 2  are exemplified by alkyl groups, such as, methyl, ethyl and propyl groups, and cycloalkyl, alkenyl, cycloalkenyl, aryl, halogen-substituted hydrocarbon and cyano-substituted hydrocarbon groups; the hydrolyzable atoms or groups denoted by X besides a hydroxy group are exemplified by alkoxy groups, such as, methoxy and ethoxy groups, acyloxy groups, such as, acetoxy groups, aminoxy and oxime groups. 
     Illustrative of the mercapto-containing silanes or siloxanes useful in the composition of the invention are the following. They are used either singly or in combination of two or more. ##STR1## 
     Next, the titanic acid esters as the second component to form the primer compositions of the present invention are exemplified by the following examples. Tetraethyl titanate, tetraisopropyl titanate, tetra(2-ethylhexyl) titanate, tetraphenyl titanate, tetra(2-methoxyethoxy) titanate and diisopropyldiacetoxy titanate and condensates of the partial hydrolyzates or cohydrolyzates of these titanates. They are employed either singly or in combination of two or more. 
     The above described first and second components of the primer composition in accordance with the present invention are employed in the proportions of from 2 to 98 parts by weight, preferably from 10 to 90 parts by weight of the former to from 98 to 2 parts by weight, preferably from 90 to 10 parts by weight of the latter. 
     The primer compositions of the present invention are applied to surfaces preferably in the form of dilutions with one or more solvents selected from the group consisting of alcohols, such as, methyl, ethyl and isopropyl alcohols, aromatic hydrocarbons such as, benzene, toluene and xylene, chlorinated, hydrocarbons, such as, trichloroethylene and perchloroethylene, aliphatic hydrocarbons, such as, hexane, ketones, such as, acetone and methylethyl ketone, and esters, such as, ethylacetate. 
     The amount of the composition of the present invention to be applied to substrate should be 5 g per m 2  of coating or more, preferably, 10 g per m 2  of coating or more, as the total of the first and second components. 
     The compositions of the present invention are useful as an effective primer when a room-temperature-curing silicone elastomer is to be bonded to and cured on surfaces of metals, such as, iron, aluminum, copper, zinc, lead, stainless steel and brass, plastics, such as, epoxy resins, vinyl chloride resins and polyester resins, and inorganic materials, such as, glass, mortar and asbestos. 
     The following examples are given for purposes of illustration to aid in understanding the invention and it is to be understood that the invention is not limited to the specific materials or operating conditions disclosed. Parts in the examples are all parts by weight. 
    
    
     EXAMPLE 1 
     Primer compositions A, B and C were prepared by mixing 3-mercaptopropyltrimethoxysilane, a titanic acid ester, and dried hexane in the different proportions as indicated in Table I. 
     
                       Table I                                                     
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                     Primer Composition                                   
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                     A    B      C                                        
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3-Mercaptopropyltrimethoxysilane,                                         
parts                  6      8      8                                    
Titanic acid ester:                                                       
Tetraisopropyl titanate, parts                                            
                       4      0      0                                    
Tetrabutyl titanate, parts                                                
                       0      2      0                                    
Tetra(2-ethylhexyl) titanate, parts                                       
                       0      0      2                                    
Hexane, parts          90     90     90                                   
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     Each primer composition was applied on a substrate of mortar formed in accordance with JIS (Japanese Industrial Standard) R 5201 as the base with its surface coated with an epoxy resin (Sanyu Resin L-270, product of Sanyu Resin Co. Ltd., Japan) and cured for 3 days at room temperature in a coating amount of 100 g/m 2 , followed by air-drying for 30 minutes. Thereupon, a room-temperature-curing silicone elastomer composition of the dehydroxylamination type (KE 70, product of Shin-Etsu Chemical Co. Ltd., Japan) was applied on the thus prepared substrates, which were then allowed to cure at room temperature for 7 days. 
     An adhesion test was carried out in accordance with JIS A 5755, where the pulling velocity was 100 mm/minute. The results of the test are set out in Table II. 
     For comparison, a similar testing piece was prepared without the application of any primer, and subjected to the same test. The results are also given in the Table. 
     
                       Table II                                                    
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               Primer Composition                                         
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               A    B       C       None                                  
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Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                 5.3    5.4     5.2   2.1                                 
Elongation at break, %                                                    
                 783    783     750   180                                 
State of rupture *      *       *     **                                  
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 *Rupture occurring in the silicone rubber.                               
 **Rupture occurring at the interface.                                    
 
    
     EXAMPLE 2 
     A primer composition D was prepared by mixing 7 parts of 3-mercaptopropyltrimethoxysilane, 3 parts of tetraisopropyltitanate, and 90 parts of dried hexane. 
     With this primer composition, a test piece was prepared and tested in the same manner as in Example 1 and also after having the test pieces elongated by 50% and immersed in sea water kept at 70° C for 4 days. The test results are shown in Table III. 
     For comparison, similar test pieces were prepared without the application of any primer or with the application of the under-mentioned primer compositions I, II and III. The test results are also given in the Table. 
     
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Primer Composition I                                                      
3-Aminopropyltriethoxysilane                                              
                     7 parts                                              
Tetraisopropyl titanate                                                   
                     3 parts                                              
Hexane               90 parts                                             
Primer Composition II                                                     
N-(2-aminoethyl)-3-amino-                                                 
propylmethyldimethoxysilane                                               
                     7 parts                                              
Tetrabuthyl titanate 3 parts                                              
Hexane               90 parts                                             
Primer Composition III                                                    
N-(2-aminoethyl)-3-amino-                                                 
propylmethyldimethoxysilane                                               
                     7 parts                                              
Tetra(2-ethylhexyl) titanate                                              
                     3 parts                                              
Hexane               90 parts                                             
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                       Table III                                                   
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               Primer Composition                                         
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               D    I      II     III  NOne                               
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In &#34;as is&#34; state:                                                         
Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                 5.7    4.6    3.6  4.5  2.2                              
Elongation at break,%                                                     
                 1110   1020   830  1030 190                              
Modulus at 150%                                                           
elongation, kg/cm.sup.2                                                   
                 1.9    1.9    1.9  2.0  1.9                              
State of rupture *      *      *    *    **                               
Afer immersion in sea water:                                              
Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                 4.8    2.4    --   --   --                               
Elongation at break,%                                                     
                 1210    670   --   --   --                               
Modulus at 150%                                                           
elongation, kg/cm.sup.2                                                   
                 1.3    1.4    --   --   --                               
State of rupture *      **     **   **   **                               
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 *Rupture occurring in the silicone                                       
 **Rupture occurring at the interface                                     
 
    
     EXAMPLE 3 
     A test piece was prepared in the same manner as in Example 1 by using a glass plate as the substrate and the primer composition D of Example 2, and tested for the various properties, with the results as set out in Table IV. 
     For comparison, a similar test piece was prepared without the use of any primer composition and tested similarly with the results as given in the same Table. 
     
                       Table IV                                                    
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                    Primer Composition                                    
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                    D      None                                           
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In &#34;as is&#34; state:                                                         
Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                      4.6      4.4                                        
Elongation at break,% 1300     1210                                       
Modulus at 150%                                                           
elongation, kg/cm.sup.2                                                   
                      1.6      1.6                                        
State of rupture      *        **                                         
After immersion in sea water:                                             
Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                      4.1      --                                         
Elongation at break,% 1180     --                                         
Modulus at 150%                                                           
elongation, kg.cm.sup.2                                                   
                      1.5      --                                         
State of rupture      *        --                                         
After immersion in deionized water:***                                    
Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                      4.8      --                                         
Elongation at break,% 1330     --                                         
Modulus at 150%                                                           
elongation, kg/cm.sup.2                                                   
                      1.5      --                                         
State of rupture      *        **                                         
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 *Rupture occurring in the silicone rubber.                               
 **Rupture occurring at the interface.                                    
 ***After having the test piece immersed in deionized water kept at       
 95° C for 24 hours.                                               
 
    
     EXAMPLE 4 
     The same procedure as in Example 3 was taken except that an aluminum or stainless steel plate was used as the substrate instead of glass plate. The test results are given in Table V. 
     
                       Table V                                                     
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                           Stainless                                      
                   Aluminum                                               
                           steel                                          
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In &#34;as is&#34; state:                                                         
Tensile strength,kg/cm.sup.2                                              
                     5.0       5.5                                        
State of rupture     *         *                                          
After immersion in sea water:                                             
Tensile strength, kg/cm.sup.2                                             
                     7.4       5.9                                        
State of rupture     *         *                                          
After immersion in deionized water:                                       
Tensile strength,kg/cm.sup.2                                              
                     7.4       5.8                                        
State of rupture     *         *                                          
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 *Rupture occurring in the silicone rubber.                               
 
    
     EXAMPLE 5 
     Test pieces were prepared in the same procedure as in Example 3 except only that the substrates as indicated in Table VI were used instead of glass plate. These test pieces in an &#34;as is&#34; state were tested for adhesion, with the results shown in the Table. 
     For comparison, similar test pieces were prepared without the use of any primer, and subjected to tests with the results shown in the same Table. 
     
                       Table VI                                                    
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                    Primer Composition                                    
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                    D     None                                            
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Copper                o       x                                           
Brass                 0       x                                           
Galvanized iron       o       x                                           
Steel                 o       x                                           
Enamel                o       x                                           
Polyester resin       o       x                                           
Marble                o       x                                           
Asbestos              o       x                                           
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 Note: Mark &#34;o&#34; indicates a good adhesion while mark &#34;x&#34; indicates the    
 occurrence of peeling.                                                   
 
    
     Apart from the above tests, the similar test pieces with the primer composition D and the varied substrates were dipped in deionized water at room temperature for 4 days and then at 90° C for 24 hours, and thereafter subjected to a similar test, to find that all the test pieces exhibited good adhesion. 
     EXAMPLE 6 
     Test pieces were prepared in the same procedure as in Example 1 except that the substrates were varied as indicated in Table VII and that a room-temperature-curing silicone rubber elastomer of the deacetoxylation type (KE 42, product of Shin-Etsu Chemical Co., Ltd., Japan) or that of the deoximation type (KE 45, product of the same company) was used instead of the room-temperature-curing silicone rubber elastomer of the dehydroxylamination type. These test pieces in an &#34;as is&#34; state or after immersion in deionized water at room temperature for 4 days were tested for adhesion and the results are set out in Table VII. 
     For comparison, similar test pieces were prepared without the use of any primer compositions and tested with the results as given in the same Table. 
     
                                           Table VII                               
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                                  Stainless                               
Substrate    Glass                                                        
                  Alminum                                                 
                        Copper                                            
                             Brass                                        
                                  steel                                   
Primer       D Nil                                                        
                  D  Nil                                                  
                        D Nil                                             
                             D Nil                                        
                                  D  Nil                                  
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     In &#34;as is&#34; state                                                     
             o o                  o  x                                    
KE42                                                                      
    After immersion                                                       
             o x                  o                                       
    In &#34;as is&#34; state                                                      
                  o  x  o x  o x  o  x                                    
KE45                                                                      
    After immersion                                                       
                  o     o    o    o                                       
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     EXAMPLE 7. 
     Primer compositions E, F and G were prepared by mixing 3-mercaptopropyltrimethoxysilane, tetraisopropyl titanate and dried hexane in the proportions as indicated in Table VIII. 
     
                       Table VIII                                                  
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                Primer Composition                                        
                E      F        G                                         
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3-Mercaptopropyltrimethoxy-                                               
silane, parts     8        5        2                                     
Tetraisopropyl titanate,                                                  
parts             2        5        8                                     
Hexane, parts     90       90       90                                    
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     Then the above primer compositions were applied on the same substrates as used in Example 1, followed by the same treatment as in Example 1 to form test pieces. The test pieces were subjected to the adhesion test with the results shown in Table IX. 
     For comparison, similar test pieces were prepared without the use of any primer compositions for purposes of a similar test. The results are also given in the same Table. 
     
                       Table IX                                                    
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              Primer Composition                                          
              E     F       G       None                                  
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In &#34;as is&#34; state                                                          
                o       o       o     x                                   
After immersion in sea                                                    
water kept at 70° C for                                            
4 days          o       o       o     x                                   
After immersion in de-                                                    
ionized water at room                                                     
temperature for 4 days                                                    
                o       o       o     x                                   
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 Note:                                                                    
 Mark &#34;o&#34; indicates a good adhesion, while mark &#34;x&#34; indicates the         
 occurrence of peeling.                                                   
 
    
     EXAMPLE 8 
     A mixture of 36.0 g of 3-mercaptopropylmethyldimethoxy silane and 58.8 g of 3-mercaptopropyltrimethoxy silane was dropped into a mixture of 5.0g of water and 100 g of dioxane. After the resulting mixture was stirred at 70° C for 3 hours to cause partial hydrolysis reaction, the solvent was distilled off together with the methyl alcohol formed by the hydrolysis of the silanes, to produce 86.5 g of a siloxane product having mercaptopropyl groups in its molecules as the partial hydrolyzate-condensate. To 6 g of this siloxane product were added 4 g of tetrabutyl titanate and 90 g of dried hexane to prepare a primer composition H. 
     The same procedures as in Example 1 for the preparation of a test piece and the carrying out of an adhesion test except only that the above-obtained primer composition H was used instead of the primer composition A, B or C. The test results are set out in Table X. 
     For comparison, a similar test piece was prepared without the use of any primer, and subjected to the adhesion test. The comparative test results are also given in the Table. 
     
                       Table X                                                     
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                 Primer Composition                                       
                 H        None                                            
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In &#34;as is&#34; state:                                                         
Adhesive strength by                                                      
pulling, kg/cm.sup.2                                                      
                   5.6        2.1                                         
Elongation at break, %                                                    
                   1030       190                                         
State of rupture   *          **                                          
After immersion in sea                                                    
water kept at 70° C for                                            
4 days                                                                    
Adhesive strength                                                         
by pulling, kg/cm.sup.2                                                   
                   4.6        --                                          
Elongation at break,%                                                     
                   960        --                                          
State of rupture   *          --                                          
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 *Rupture occurring in the silicone rubber.                               
 **Rupture occurring at the interface.