Polyurethane elastomer produced by reaction between a triazole-stabilized isocyanate prepolymer and a diamine
BACKGROUND OF THE INVENTION 
     Urethane elastomers are produced in large quantities from various isocyanate terminated prepolymers by reaction with a hindered diamine. Historically, 4,4&#39;-methylene bis (o-chloroaniline) (hereinafter &#34;MOCA&#34;) has probably been the most widely used hindered amine. However, MOCA is a carcinogen suspect agent, and, as a consequence, taking the necessary safety precautions to safeguard the health of workers substantially increases the cost involved in producing elastomers using MOCA as the hindered amine. Another hindered amine that has also seen considerable use is 4,4&#39;-methylene bis (methyl-anthranilate) (hereinafter &#34;MBMA&#34;). 
     So far as is known, unhindered diamines have not heretofore been used for reaction with an isocyanate terminated prepolymer to produce an elastomer. Pot life is short, only a few minutes, even when MOCA and MBMA are used as the diamines. In a recent experiment wherein an attempt was made to produce a urethane elastomer from an isocyanate terminated prepolymer and an unhindered diamine the pot life was estimated to be five seconds; the specific unhindered diamine used was 4,4&#39;-methylene dianiline (hereinafter &#34;MDA&#34;). 
     BRIEF DESCRIPTION OF THE INSTANT INVENTION 
     The present invention is based upon the discovery that an isocyanate terminated prepolymer can be stabilized by reaction with benzotriazole* or a tolyl triazole, and that a mixture of the stabilized prepolymer with a diamine has a longer pot life than does a mixture of the unstabilized prepolymer with that diamine. When a hindered diamine is used in producing a urethane elastomer according to the invention, e.g. MOCA or MBMA, the pot life is increased substantially so that the labor required for carrying out the mixing operation can be minimized, and the prepolymer is preferably stabilized with from 0.1 to 0.5 equivalent of the triazole per equivalent of isocyanate in the prepolymer. On the other hand, when the diamine is unhindered, urethane elastomers which, so far as is known, had not been made are the result of practicing the instant invention, and it is usually preferred that the prepolymer be stabilized with from 0.5 to 1.0 equivalent of the triazole per equivalent of isocyanate in the prepolymer. 
    
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following Examples, which are presented solely for the purpose of illustrating and disclosing the invention, set forth the best presently known mode. 
     EXAMPLE 1 
     An isocyanate-terminated prepolymer was prepared from 1 mole of a poly(oxytetramethylene)glycol having a molecular weight of 1000 and a melting temperature of 50° C. (hereinafter &#34;PTMG 1000&#34;) and 2 moles of tolylene diisocyanate (hereinafter &#34;TDL&#34;). The specific TDL used was an 80/20 blend of 2,4- and 2,6-isomers. The isocyanate (hereinafter &#34;NCO&#34;) content of the prepolymer, determined by di-n-butyl amine titration, was 6.2 percent. Benzotriazole (hereinafter &#34;BT&#34;) was then melted, vacuum degassed, and mixed with a sample of the prepolymer which had been preheated to 75° C. The BT was added in the proportion of 0.25 equivalent (mole) thereof per isocyanate equivalent of the prepolymer. The resulting mixture was stirred at 80° C. for 1 hour to insure complete reaction between the BT and the free NCO groups of the prepolymer. The stabilized prepolymer which resulted, at 80° C., was mixed with MBMA, which had been vacuum degassed at 150° C., mixing temperature 140° C., at an NCO: NH 2  ratio of substantially 1:1. The resulting composition was found to have a pot life of 20 minutes at 100° C.; it was vacuum degassed and poured into a mold preheated to 100° C. The mold, which had a central cavity 6 inches by 6 inches by 0.06 inch in depth, was then covered by a flat plate and placed in a hydraulic press where a compressive force of 10,000 pounds per square inch was applied to the mold and plate. After approximately one hour in the mold at 100° C. the partially cured elastomer was removed from the mold and transferred to an oven where it was cured for an additional 19 hours at 100° C. 
     After conditioning at room temperature and 50 percent relative humidity for one week the elastomer was found to have the following physical properties: 
     
         __________________________________________________________________________
                       Test Procedure                                     
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100% modulus, pounds per square inch,  1260                               
300% modulus, pounds per square inch,  2500                               
                       ASTM-D-412                                         
Tensile strength, pounds per square inch,                                 
                                       4380                               
Elongation at break, percent           440                                
A                                      92                                 
Shore hardness         ASTM-D-2240                                        
D                                      47                                 
Graves tear strength, pounds per inch                                     
                     } ASTM-D-624      400                                
                                       248                                
Bashore rebound, percent                                                  
                       Bashore rebound tester                             
                                       24                                 
2 percent              duPont 950 Thermogravimetric                       
                                       250                                
Thermal degradation, ° C.                                          
10 percent             Analyzer        305                                
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     Other urethane elastomers according to the invention, and controls in which no triazole was used, have been produced by the method described above in Example 1. The starting materials used, the ratio of equivalents of triazole used to equivalents of NCO, the mixing temperatures, the curing conditions, the pot life and the physical properties of the various elastomers, determined by the tests identified above, are set forth in Table 1, below. The following abbreviations, all used in Table 1, have the indicated meanings: 
     
         ______________________________________                                    
PTMG 650:   A poly (oxytetramethylene)glycol                              
            having a molecular weight of                                  
            650 and a melting point of 30° C.                      
PPG 1010:   A poly(oxypropylene)glycol having                             
            a molecular weight of 1030.                                   
H.sub.12 MDI:                                                             
            4,4&#39;-dicyclohexylmethane                                      
            diisocyanate                                                  
m-PDA:      m-phenylene diamine                                           
1,3-BAC:    1,3-bis(aminomethyl)cyclohexane                               
MXDA:       m-xylylene diamine                                            
IPDA:       isophorone diamine                                            
TT:         toyltriazole; the material used was                           
            a blend of approximately equal parts                          
            of 4-methylbenzotriazole and 5-                               
            methylbenzotriazole                                           
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     The curing was for approximately 1 hour in a mold at the temperature reported in Table 1 followed by oven curing at that temperature for the remainder of the time for each elastomer. In each case, the prepolymer was produced by reacting two moles of the indicated isocyanate with one mole of the polyol. 
     
                                           TABLE 1                                 
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                                       Mixing Temperature, ° C.    
                               Triazole/NCO                               
                                       Prepolymer                         
                                              Diamine +                   
Example                                                                   
       Polyol  Isocyanate                                                 
                     Triazole                                             
                          Diamine                                         
                               Equivalent                                 
                                       + Triazole                         
                                              Prepolymer                  
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2      PTMG 1000                                                          
               TDI   BT   MBMA 0.50    80     140                         
Control* (1,2)                                                            
       PTMG 1000                                                          
               TDI   None MBMA --      --     140                         
3      PTMG 1000                                                          
               TDI   TT   MBMA 0.25    90     140                         
4      PTMG 1000+                                                         
               TDI   TT   MBMA 0.25    90     120                         
       PTMG 650**                                                         
5      PTMG 1000                                                          
               TDI   TT   MOCA 0.25    80     110                         
Control*(5)                                                               
       PTMG 1000                                                          
               TDI   None MOCA --      --     110                         
6      PTMG 1000                                                          
               TDI   TT   MOCA 0.25    80     110                         
7      PTMG 1000                                                          
               TDI   TT   MDA  0.5     60     90                          
Control*(7)                                                               
       PTMG 1000                                                          
               TDI   None MDA  --      --     90                          
8      MTMG 1000                                                          
               TDI   TT   MDA  1.0     60     90                          
9      PTMG 1000                                                          
               TDI   BT   m-PDA                                           
                               0.5     40     70                          
10     PTMG 1000                                                          
               H.sub.12 MDI                                               
                     BT   m-PDA                                           
                               0.27    90     80                          
11     PPG     H.sub.12 MDI                                               
                     TT   1,3 BAC                                         
                               0.5     60     25                          
12     PPG     H.sub.12 MDI                                               
                     TT   MXDA 0.5     60     25                          
13     PPG     H.sub.12 MDI                                               
                     TT   IPDA 0.5     60     25                          
14     PPG     H.sub.12 MDI                                               
                     TT   IPDA 1.0     60     25                          
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 *Not according to the present invention; presented for purposes of       
 comparison with example(s) in parentheses.                               
 **Equimolecular proportions.                                             
 
    
     
                            Pot   100% 300% Tensile                                
                                        Elongation                        
                                               Shore                      
       Cure Conditions                                                    
                   Life  Modulus,                                         
                              Modulus,                                    
                                   Strength                               
                                        at     Hardness                   
Example                                                                   
       Time (hrs)                                                         
             Temp (° C.)                                           
                   Min/° C.                                        
                         Psi  Psi  Psi  Break, %                          
                                               A D                        
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2      20    100   30/100                                                 
                         1200 1900 3050 460    92                         
                                                 45                       
Control*(1,2)                                                             
       20    100    4/100                                                 
                         2022      4043 231    94                         
                                                 50                       
3      20    100   22/140                                                 
                         1667 2318 4263 360    92                         
                                                 46                       
4      20    110   30/120                                                 
                          770 2283 4650 378    88                         
                                                 45                       
5      20    100    9/110                                                 
                         1745 2740 4820 479    93                         
                                                 45                       
Control*(5)                                                               
       20    100    4/110                                                 
                         1987 3465 4871 383    96                         
                                                 55                       
6       3    125   10/110                                                 
                         1700 2400 4600 488    94                         
                                                 48                       
7       3    125    3/90 1126 1824 4500 520    95                         
                                                 46                       
Control*(7)                                                               
       --    --    5 sec/90**                                             
                         --   --   --   --     --                         
                                                 --                       
8      2.5   125   15/90   650                                            
                              1000 3020 640    87                         
                                                 35                       
9      20    100    4/50  924 1389 4151 600    92                         
                                                 43                       
10     22    110   0.5/80                                                 
                          690 1380 3070 515    78                         
                                                 36                       
11      3    100   0.5/60                                                 
                          573 1650 3679 462    72                         
                                                 ND***                    
12     18    100   0.5/60                                                 
                          380 1216 3369 508    71                         
                                                 ND                       
13     19    100   0.5/60                                                 
                          636 2419 4019 390    74                         
                                                 ND                       
14     18     90    1/60  419 1201 2957 485    74                         
                                                 ND                       
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 *Not according to the present invention; presented for purposes of       
 comparison with example(s) in parentheses.                               
 **The pot life was too short for processing.                             
 ***Not determined.                                                       
 
    
     
         Graves                      Rebound,   TGA, ° C.                   
Example       Tear Strength, pi                                           
                            %          2%      10%                        
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2             400           29         225     296                        
Control*(1,2) 364           ND**       250     295                        
3             388           ND         ND      ND                         
4             370           ND         ND      ND                         
5             462           ND         ND      ND                         
Control*(5)   489           ND         ND      ND                         
6             480           ND         ND      ND                         
7             390           ND         ND      ND                         
Control*(7)   --            --         --      --                         
8             410           ND         ND      ND                         
9             392           ND         ND      ND                         
10                          ND         ND      ND                         
11            247           ND         240     312                        
12            187           ND         240     322                        
13            270           ND         235     309                        
14            203           ND         235     305                        
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 *Not according to the present invention; presented for purposes of       
 comparison with example(s) in parentheses.                               
 **Not determined.                                                        
 
    
     EXAMPLES 15-17 
     Substantially the procedure described above in Example 1, except that the prepolymer:diamine blend was merely poured into a pan mold after degassing, was used to produce urethane elastomers according to the method of the invention from a prepolymer made from TDL and a polyester which was hydroxy-terminated. The specific prepolymer had an NCO content of approximately 3.5 percent by weight; it is commercially available under the trade designation &#34;Vibrathane 6025&#34;. Data concerning the elastomers are set forth in Table 11, below: 
     
                                           TABLE 11                                
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                           Mixing Temperature ° C.                 
                  Triazole/NCO                                            
                           Diamine +                                      
Example                                                                   
       Triazole                                                           
             Diamine                                                      
                  Equivalent                                              
                           Prepolymer                                     
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15     TT    MOCA 0.5      100                                            
16     TT    MDA  1.0      100                                            
17     TT    m-PDA                                                        
                  1.0      100                                            
Control*(15)                                                              
       --    MOCA --       100                                            
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 *Not according to the present invention; presented for purposes of       
 comparison with the example in parentheses.                              
 
    
     
                            Pot   Tensile                                          
                              Elongation                                  
                                    Shore                                 
       Cure Conditions                                                    
                   Life  Strength                                         
                              at    Hardness                              
Example                                                                   
       Time(hrs)                                                          
             Temp(° C.)                                            
                   Min/° C.                                        
                         Psi  Break, %                                    
                                    A                                     
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15     16    107   13    3760 550   80                                    
16     16    107   9     3210 590   78                                    
17     16    107   9     3870 600   80                                    
Control*(15)                                                              
       16    100   7     3640 685   80                                    
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 *Not according to the present invention; presented for purposes of       
 comparison with example(s) in parentheses.                               
 
    
     It is known e.g., in U.S. Pat. No. 3,721,645, that triazoles other than BT and TT are capable of reaction with NCO groups, and that the reaction is reversible in the sense that apparently normal cure occurs at an elevated temperature. It will be apparent that an equivalent amount of any of the other triazoles could be substituted for BT and TT in each of the foregoing Examples, and with similar results. However, BT and TT are the preferred triazoles because of availability, low volatility, anti-corrosive properties and price. 
     The curing times reported in the foregoing Examples are those which were actually used. It has been found, however, that the 18 to 20 hour curing times used in many instances are unnecessary in the sense that substantially the same physical properties result after a shorter cure. In most cases, a cure time of three to six hours has been found to be adequate. 
     Various changes and modifications can be made, as will be apparent to one skilled in the art, from the specific embodiments described above without departing from the spirit and scope of the invention as defined in the following claims: