Patent Publication Number: US-4925504-A

Title: Ambient cure catalyst for solid propellants

Description:
DEDICATORY CLAUSE 
     The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalties thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     The need for compounds for use in propellant compositions to prevent catalysis of the urethane reaction by cure inducing materials such as Catocene, iron oxide, ferric fluoride, etc. without interfering with the function of the cure catalyst triphenylbismuthine was met with the employment of about 0.02 to about 0.03 weight percent of dicarboxylic acids (e.g. maleic and oxalic or anhydrides of the same). Extended potlife in diisocyanate cured polymer systems, such as hydroxyl-terminated polybutadienes, polyesters, etc. is achieved with typical potlife changes being increased from 0-2 hours to 12-15 hours. The use of triphenylbismuthine (TPB) promotes the urethane reaction as well; therefore, the use of the dicarboxylic acid provides other advantages which enables TPB to be used even with the more reactive isocyanates since potlife is extended and undesirable side reactions are reduced as well. A commonly assigned U.S. Pat. No. 4,597,811, titled: &#34;Prevention of Unwanted Cure Catalysis in Isocyanate Cured Binders&#34;, which was issued to Marjorie E. Ducote on July 1, 1986, provides additional teachings of cure catalysis behavior of TPB and the dicarboxylic acids, including the discovery of the additional benefits achieved from premixing the catalyst system with the diisocyanate curing agent before they are added to the propellant slurry. This behavior was evidenced in the use of maleic anhydride (MAN) particularly where maleic acid (MAC) is present as a contaminant in the MAN and where MAC is produced by hydrolysis of MAN. 
     SUMMARY OF THE INVENTION 
     The benefit of a fast-acting cure catalyst for use in hydroxyl-terminated polybutadiene-based solid propellant formulation which can be derived by polymerization at ambient temperature (80° F.) cure or by accelerated oven (140° F.) cure are readily recognized. This benefit is achieved by the use from about 0.015 to about 0.025 weight percent additive of tris(ethoxyphenyl)bismuthine (TEPB) in place of triphenylbismuthine (TPB). The compound tris(ethoxyphenyl)bismuthine is synthesized by the reaction of ethoxyphenylmagnesium bromide with bismuth trichloride. 
     The evaluation of TEPB included comparing the characteristics and mechanical properties derived from its use in an ambient mixed and cured propellant with a high temperature-cured propellant composition using TPB as the cure catalyst additive. The results indicate that the ambient cure process is viable and produces a propellant having adequate mechanical properties. The control and experimental propellant employed the same propellant ingredients in the amounts set forth under Table I below. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Tris(ethoxyphenyl)bismuthine is used in a hydroxyl-terminated polybutadiene-based solid propellant formulation at ambient mixing and curing at 80° F. and in accelerated oven cure at 140° F. This use is very cost effective while meeting or exceeding the 30% thermal strain and 40% cold ignition strain values and also having acceptable processibility. Table I below depicts the propellant formulation used to evaluate the high temperature curing with TPB and the ambient mixing and curing with TEPB. 
     
                       TABLE I                                                     
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Evaluation of the Ambient Mixed and Cured                                 
Propellant with the High Temperature-Cured Propellant                     
              HIGH-TEMP-   AMBIENT                                        
              ERATURE      MIXED AND                                      
              CURED        CURED                                          
Composition/  PROPELLANT*  PROPELLANT**                                   
Characteristics                                                           
              (Wt %)       (Wt %)                                         
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Composition                                                               
Ammonium perchlorate                                                      
              29.5         29.5                                           
(%) (400-micrometers)                                                     
Ammonium perchlorate                                                      
              30.0         30.0                                           
(%) (200-micrometers)                                                     
Ammonium perchlorate                                                      
              5.0          5.0                                            
(%) (20-micrometers)                                                      
Aluminum Powder (%)                                                       
              14.0         14.0                                           
HMX*** (%)    10.0         10.0                                           
(4-micrometers)                                                           
Hydroxyl-terminated                                                       
              11.5         11.5                                           
polybutadiene polymer                                                     
(%)                                                                       
Isophorone diisocyanate                                                   
Isocyanate/hydroxyl                                                       
              0.89         0.92                                           
ratio                                                                     
Characteristics                                                           
End-of-mix Viscosity                                                      
4 hrs (KP)    8.7          11.1                                           
8 hrs (KP)    25.7                                                        
Shore A Hardness                                                          
              40                                                          
Burning Rate (r.sub.1000)                                                 
              0.26         0.28                                           
(ips) (cured)                                                             
Burning Rate (r.sub.1000)                                                 
              0.25         0.27                                           
(ips) (cured)                                                             
Pressure Exponent                                                         
              0.30         0.30                                           
Mechanical Properties                                                     
(2 ipm) (77° F.)                                                   
Tensile Strength (psi)                                                    
              56           94                                             
Strain @ Max Stress (%)                                                   
              26           42                                             
Modulus (psi) 262          400                                            
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 *Contains Triphenylbismuthine 0.025 weight percent                       
 **Contains Tris(ethoxyphenyl)bismuthine 0.025 weight percent additive.   
 ***HMX is cyclotetramethylenetetranitramine, an organic oxidizer.        
 
    
     A series of pint mixes were also made in which the stoichiometry of the ingredients were varied. This showed that an ambient cure process is viable, and the process produces a propellant having adequate mechanical properties. 
     The evaluations above show that the mechanical properties and characteristics meet or exceed the values for an acceptable propellant formulation during processing and after curing. 
     Both oven and ambient cures for the TEPB propellant were found to be more cost effective than the TPB propellant employing conventional oven cure. In addition, cost calculations on propellant manufacture which compared the conventional oven cure with the accelerated oven cure and the ambient cure reveal that the ambient cure was the most cost effective. 
     A comparison of the effect of triphenylbismuthine and tris(ethoxyphenyl)bismuthine assuming the production of 100  booster motors for an advanced exoatmospheric interceptor is presented in TABLE II. These calculations effectively prove that a major cost savings can be achieved through the use of tris(ethoxyphenyl)bismmuthine instead of tripheylbismuthine, and additionally, a marked shortening of the process time is achieved. 
     
                       TABLE II                                                    
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(Cost Comparison of Effect of Propellant Cure                             
Catalysts Assuming 100 Booster Motors Manufacture for an                  
Advanced Exoatmospheric Interceptor)                                      
            Conven-                                                       
            tional*                                                       
                   Accelerated**                                          
                               Ambient**                                  
            Oven   Oven        Oven                                       
            Cure   Cure        Cure                                       
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Processing Time                                                           
               9        5           7                                     
(days) (total)                                                            
Potlife (hours)                                                           
              20        8          20                                     
Mix time (first stage                                                     
               6        5           6                                     
motors) (hours)                                                           
Mix time (second stage                                                    
              12       10          12                                     
motors) (hours)                                                           
First-stage mold sets                                                     
              30       20          24                                     
required (No.)                                                            
Second-stage mold sets                                                    
              36       20          24                                     
required (No.)                                                            
Amortized mold sets                                                       
              $168,000 $100,000    $120,000                               
costs/motor                                                               
Heating cost/motor                                                        
               $19,800  $12,000    0                                      
Pressure costs/motor                                                      
                6,600   $4,000     0                                      
Chemical***    $28,000 0           0                                      
modification                                                              
costs/motor                                                               
Mixing costs/motor                                                        
              $182,000 $210,000    $182,000                               
Total costs/motor                                                         
              $404,000 $326,000    $302,000                               
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 *Contains triphenylbismuthine                                            
 **Contains tris(ethoxyphenyl)bismuthine                                  
 ***Hydroxyl-terminated polybutadiene prepolymer modified by chain        
 extension