Resin system for filament winding of pressure vessels
BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     The present invention relates to a resin system and more particularly to a resin system for filament winding of pressure vessels. 
     Description of the Prior Art. 
     In building filament wound high strength pressure vessels, such as for production rocket motors, it is necessary to use a resin system including binders and curing agents that are workable during the filament winding process and then properly harden to seal and retain the filaments in place. The filament material, for example, Kevlar, preferably has low weight, high strength, high temperature resistance and inert characteristics. Several different characteristics are needed in a resin system used for the complex filament winding of large pressure vessels for rocket motors. One such characteristic is that it have a long working life. Another is that it have suitable viscosity during the working process. Still another is that it cure properly after having been fully wound. It has been found that no single epoxy has all of the required characteristics. 
     The present invention overcomes these problems by providing a resin system having a plurality of different epoxies and curing agents that provide improved working and strength characteristics for the manufacture of complex filament wound pressure vessels. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention comprises an epoxy resin system for use in the wet winding of high strength filament wound pressure vessels with aramide fiber. The resin system is composed of a mixture of a plurality of distinct epoxy resins and aromatic curing agents. The epoxy resins were selected to impart particular properties required for the filament winding process and to improve the strength of filament wound pressure vessels. Aromatic amine curing agents were selected to alter the time temperature gel and cure conditions of the system without significantly altering the final strength of the pressure vessels. The resin system is nominally composed of a mixture of a commercially available epoxy resins comprising diglycidyl ether of bisphenol A, a diglycidyl ester of linoleic dimer acid, and a diglycidyl ether of butanediol and the aromatic amine curing agents comprise a mixture of methylenedianiline and metaphenylenediamine in a 60/40 ratio by weight. The ratios of the components are selected to meet the specific processing needs and the final product requirements. 
     STATEMENT OF THE OBJECTS OF THE INVENTION 
     An object of the present invention is to provide an effective resin system for filament winding of vessels; 
     Another object of the present invention is to provide an effective resin system for use with Kevlar as the filament winding material for pressure vessels; 
     Another object of the present invention is the use of a resin system for Kevlar pressure vessels which is largely comprised of an aliphatic constituent such as a diglycidyl ester of linoleic acid dimer; 
     Still another object of the present invention is to provide a resin system comprising a plurality of epoxy resins and aromatic curing agents to provide for effective filament winding of pressure vessels; 
     Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention comprises an epoxy resin system for the wet winding of high strength filament wound pressure vessels with aramide fiber. The resin is composed of a mixture of a plurality of distinct epoxy resins and aromatic curing agents. The epoxy resins were selected to impart particular properties required for the filament winding process and to improve the strength of filament wound pressure vessels. Aromatic amine curing agents were selected to alter the time/temperature gel and cure conditions of the system without significantly altering the final strength of the pressure vessels. The resin system is nominally composed of a mixture of a commercially available epoxy resin comprising diglycidyl ether of bisphenol A, a diglycidyl ester of linoleic dimer acid, and a diglycidyl ether of butanediol and the aromatic amine curing agents comprise a mixture of methylenedianiline and metaphenylenediamine in a 60/40 ratio by weight. The ratios of the components are selected to meet the specific processing needs and the final product requirements. 
     The following are the components and the weight range of the components suitable for use as a resin system for the manufacture of rocket motor pressure vessels using filament winding techniques. 
     
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                           parts                                          
                           by                                             
                           weight                                         
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Component A                      100                                      
1.       diglycidyl ether of bisphenol A -                                
         20-30% by weight (this epoxy resin                               
         has a relatively high viscosity)                                 
2.       diglycidyl ester of linoleic dimer                               
         acid - 80-70% by weight (this                                    
         epoxy resin is relatively soft)                                  
Component B                      10-40                                    
         diglycidyl ether of butanediol                                   
         (this epoxy resin has a low viscosity but                        
         upon hardening becomes brittle and hard)                         
Component C                      20-40                                    
         A mixture of methylenedianiline and                              
         metaphenylenediamine in a 60/40 ratio by                         
         weight. (this is a curing agent)                                 
Filament                                                                  
         Kevlar 49, manufactured by DuPont                                
         Corporation under this name and made                             
         commercially available.                                          
         The characteristics                                              
         of this material are:                                            
         denier - 4440-4640                                               
         density - 1.42-1.48 gm/cc                                        
         tensile strength - 250,000 psi min.                              
         modulus - 17.5 × 10.sup.6 min.                             
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     A more complete chemical definition of the component A and B epoxies may be found in Chapter 4 and Appendices 1 and 2 of the Handbook of Epoxy Resins, Lee and Neville, McGraw-Hill, Copyright 1967. A more complete chemical definition of Kevlar 49 may be found in &#34;Textile Research Journal&#34;, pp. 62-66, published January 1977. 
     The normal time for winding rocket motor pressure vessels may take as long as seven days. The filament winding process comprises the use of a minimum friction winding path as defined by the geometry of the pressure vessel. For purpose of manufacturing it has been found that the resin system should have about the following characteristics: 
     viscosity--1,200 centipoise at 25° C. 
     gel time--18 hours with a viscosity increase not in excess of 5,000 centipoise at 25° C. 
     complete gel set--24 hours at 25° C. 
     cure--120° C. for 4 hours 
     meets suitable pressure vessel performance testing requirements. 
    
    
     EXAMPLE I 
     
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             parts by weight                                              
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Component A   100 pbw                                                     
Component B    14 pbw                                                     
Component C    17 pbw                                                     
Filament      500 pbw                                                     
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     EXAMPLE II 
     
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 Component A  100 parts by wt.                                            
Component B    25 parts by wt.                                            
Component C    30 parts by wt.                                            
Filament      500 parts by wt.                                            
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     Both examples had the following characteristics: 
     Viscosity--about 1,200 centipoise at 25° C. 
     Gel time--about 18 hours-viscosity increase no more than about 5,000 centipoise at 25° C. 
     Complete gel set--about 24 hours 
     Cure--120° C. for about 4 hours 
     Performance of both examples in standard pressure vessels were satisfactory. 
     
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Pressure Vessel Testing                                                   
             Burst    Helical  Hoop                                       
             Strength Stress   Stress                                     
Composition  (psi)    (Ksi)    (Ksi)  PV/W                                
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Prior resin system                                                        
             2807     344.1    342.8  1.33                                
Example I    2949     361.4    360.1  1.375                               
Example II   2900     350      350    1.50                                
             (approx.)                                                    
                      (approx.)                                           
                               (approx.)                                  
                                      (approx.)                           
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