In depth curing of resins induced by UV radiation
BACKGROUND OF THE INVENTION 
     In order to reduce the amount of energy needed for polymerizing vinyl monomers and for evaporating and burning solvents they are in, various types of radiation cures have been tried. Ionizing radiation, such as X-rays, randomly knocks off atoms and groups producing a highly cross-linked polymer. However, because ionizing radiation is difficult to handle safely, it has not been widely used. 
     Ultraviolet radiation has also been used in combination with a photo-initiator which initiates the polymerization; but because of the absorption characteristics of materials, the light can only penetrate to a depth of about 20 mils. When the monomer is pigmented or opaque materials are present in the resin, the depth of cure is even less. 
     SUMMARY OF THE INVENTION 
     I have found that if an accelerator is present in the monomer composition, a photo-initiator and a thermal initiator can act synergistically with UV light so that the depth of cure achieved is much greater than when either are used alone. My invention has resulted in curing at depths of 500 mils or greater. 
     A great advantage of this invention is that no external source of heat, other than the UV light, is required for the cure. Curing ovens and the natural gas they use for heating are not needed. 
     Another important advantage of this invention is that no solvent is used in the final composition. Since solvents pollute the air when they evaporate and therefore must be burned, energy is saved in both the evaporation and incineration of solvent. Also, a solvent can sometimes produce bubbles or blisters in the product when it volatilizes, and this is also avoided. 
     Finally, because the thermal initiator completes the polymerization begun by the photo-initiator, the monomer is cured even when pigmented or when opaque material is present. Also, because the subsequent thermal reaction is omnidirectional, the resin will cure in shadowed areas thus overcoming the line-of-sight limitation normally associated with radiation curing. 
     PRIOR ART 
     U.S. Pat. Nos. 3,948,740, 3,876,518, and 4,022,855 disclose the polymerization of an ethylenically unsaturated monomer composition containing peroxides or azo compounds by exposure to ionizing radiation such as X-rays. 
     U.S. Pat. No. 3,784,524 discloses UV curing of resins containing photo-initiators. 
     U.S. Pat. No. 3,937,855 discloses the UV curing of the skin of a resin to prevent dripping prior to thermal cure. 
     DESCRIPTION OF THE INVENTION 
     THE MONOMER 
     The monomer is a liquid having the general formula CH 2  ═CH--R where R is an electron withdrawing group. Acrylates are preferred because they have high reactivity, low toxicity, and result in a product having excellent physical and electrical properties. Tetraethylene glycol diacrylate (TEGDA), 2-hydroxyethyl acrylate (HEA), and hexanediol diacrylate (HDDA) are the preferred monomers because of their low viscosity, their high reactivity, and because they readily dissolve many pre-polymers to form low viscosity solutions. Trimethylolpropane triacrylate (TMPTA) is preferred for its high reactivity, although its viscosity is also higher. Ethyl methacrylate (EMA) and 2-ethylhexyl acrylate (EHA) are preferred for low viscosity, although their reactivity is also lower. In order to achieve both low viscosity and high reactivity, it is often desirable to blend various monomers having one or the other of these properties. Other monomers include allyl methacrylate (AM), acrylic acid (AA), acrylamide (AAM), acrylonitrile (AN), benzyl acrylate (BA), beta-cyanoethyl acrylate (BCEA), bis-phenol A dimethacrylate (BPADM), ethyl methacrylate (EA), ethylene glycol dimethacrylate (EGDM), neopentyl glycol diacrylate (NPGDA), styrene (STY), vinyl acetate (VA), vinyl imidazole (VI), and vinyl pyrollidone (VP). Mixtures of monomers are also contemplated. 
     OLIGOMERS 
     Oligomers are preferably included in the composition to improve the physical properties of the product. The oligomer must have at least one reactive vinyl group so that it is reactive with the monomer. Acrylic or methacrylic functionality is preferred as those oligomers have the best properties. The oligomer should not have a molecular weight greater than 10,000 because higher molecular weight oligomers are less soluble and, because more monomer is required for solution, the properties of the product are poorer. Oligomers having molecular weights up to about 2000 are preferred as they are more soluble. 
     The minimum molecular weight of most oligomers is about 400. The oligomer may be an acrylic terminated epoxy, polyester, polyurethane, polyimide, polyacrylate, alkyd, phenolic alkyd, or other similar type of oligomer. If the oligomer is a solid, it must be soluble in the monomer. 
     The oligomer may constitute up to about 90% (all percentages herein are by weight based on total composition weight) of the total composition, but less than about 20% is usually not worth the trouble. About 50 to about 80% is preferred. If large amounts of oligomers are used, it will probably have to be a liquid oligomer in order for the total composition to be a liquid. 
     Particular oligomers that have been used and have worked well are the reaction product of hydroxy ethyl acrylate and toluene diisocyanate in a 1 to 1 molar ratio, a polyester containing maleic anhydride sold by Westinghouse Electric Corp. under the trade designation &#34;BT 4487-2,&#34; and a polyester containing fumaric acid sold by Hoechst under the trade designation &#34;Alpolit 81.&#34; 
     Also useful is a resin which is made from a mixture of 2820 pbw glycerol, 2360 pbw isophthalic acid, 5750  pbw soya fatty acids, and 593 pbw maleic anhydride. The mixture is dissolved in 910 pbw 2-ethyl hexyl acrylate, 2220 pbw tetraethylene glycol diacrylate, and 4680 pbw trimethylolpropane triacrylate. Mixtures of oligomers can be used. 
     PHOTO-INITIATOR 
     The composition must contain about 0.01 to about 4% of a photo-initiator. More than about 4% is unnecessary and may make the composition unstable and less than about 0.01% is insufficient to cure the monomer. A preferred concentration is about 0.075 to about 1%. It is the purpose of the photo-initiator to initiate polymerization of the monomer when exposed to UV radiation. 
     Photo-initiators include benzoin ethers, ketones such as benzophenone, Michler&#39;s ketone, ketones accelerated with amines such as dimethyl ethanolamine, and thioxanthen-9-one derivatives. Liquid photo-initiators such as benzoin ethers and diethoxyacetophenone are preferred for ease of handling and because no solvent is needed for their dissolution. Solid photo-initiators are less preferred because they often require small amounts of a solvent for dissolution. Thioxanthen-9-one derivatives are the least satisfactory photo-initiators because of their poor solubility in solvents. Other photo-initiators include benzophenone, acetophenone quinone, methyl ethyl ketone, valero-phenone, hexanophenone, γ-phenylbutyrophenone, p-morpholinopropiophenone, dibenzosuberone, 4-morpholinobenzophenone, 4-morpholinodeoxybenzoin, p-diacetylbenzene, 4-aminobenzophenone, benzaldehyde, α-tetraline, 9-acetylphenanthrene, 2-acetylphenanthrene, 10-thioxanthenone, 3-acetylphenanthrene, 3-acetylindole, 9-fluorenone, 1-indanone, 1,3,5-triacetylbenzene, thioxanthen-9-one, xanthene-9-one, 7-H-benz[de]-anthracen-7-one, 1-naphthaldehyde, 4,5&#39;-bis(dimethylamino)-benzophenone, fluorene-9-one, 1&#39;-acetonaphthone, 2&#39;-acetonaphthone, 2,3-butanedione, and the like. Mixtures of photo-initiators are also contemplated. 
     THERMAL INITIATOR 
     The composition must contain a thermal initiator. The thermal initator initiates the polymerization of the monomer when it is heated as a result of the exothermic polymerization reaction initiated by the photo-initiator. The amount of thermal initiator should be less than about 4% as more is unnecessary and may make the composition unstable. On the other hand, less than about 0.01% is insufficient to cure the monomer. A preferred concentration is about 0.075 to about 1%. 
     Peroxides, hydroperoxides and certain azo compounds are generally suitable as thermal initiators. Examples of thermal initiators include lauroyl peroxide, cumene hydroperoxide, 2,5-dimethyl 2,5-bis(hydroperoxy)hexane, hydrogen peroxide, and methyl ethyl ketone peroxide. The preferred thermal initiators, which are more efficient, are benzoyl peroxide, dicumyl peroxide, t-butyl hydroperoxide, t-butyl perbenzoate, di-t-butyl peroxide, bis(t-butyl peroxy isopropyl)benzene, 2,5-dimethyl-2,5-bis(t-butyl peroxy)hexane, azo bis(isobutyronitrile), and cyano t-butyl azo cyclohexane. The most preferred thermal initiator is di-t-butyl peroxide because it is highly reactive, it is a liquid, and it is relatively stable, having a half life of 10 hours at 126° C. Mixtures of thermal initiators are also contemplated. 
     ACCELERATOR 
     The composition must include an accelerator to lower the activation energy of the thermal initiator. At least about 0.01% of an accelerator is needed as less is ineffective, but more than about 0.5% is unnecessary. 
     Non-aromatic accelerators are preferred as it has been found by experiment that most aromatic accelerators do not work well. Amines or amides are preferred as they are more effective. Methyl diethanolamine is the most preferred compound as it has the best reactivity. Other accelerators which are preferred as they make the polymerization reaction go to completion are diethyl formamide, p-toluidine (N,N-dimethyl), and di-propylamine. Other accelerators include dimethyl cyclohexylamine, dimethyl formamide, ethyl amino ethanol, and triethylamine. 
     Two other useful accelerators are para-toluene sulfonamide and benzoic sulfimide. These compounds contain an --NH group which accelerates the decomposition of peroxide thermal initiators. They can be used in combination with other accelerators for best effect. Mixtures of other accelerators are also contemplated. 
     FREE RADICAL SCAVENGER 
     The composition preferably contains a free radical scavenger (also called an &#34;inhibitor&#34;) to increase its shelf life. The amount of inhibitor should be at least about 0.01% as less is ineffective, but if more than about 1% is used, the polymerization reaction may not occur. The preferred inhibitor is hydroquinone as it has been found experimentally to work well. Other inhibitors include benzoquinone, cresylic acid, 2,5-dimethyl benzoquinone, and picric acid. 
     RHEOLOGICAL AGENT 
     Although preferably not included in the composition, a rheological agent may be used to change rheological properties such as thixotropy or to increase viscosity. A rheological agent should not be reactive with the monomer or other components of the composition. If a rheological agent is used, the amount of it should be at least about 1% as less is ineffective, but more than about 5% may adversely affect the physical properties of the product. Polyamide-modified alkyds may be used to increase thixotropy. Polyvinyl acetate, which is soluble in organic esters, may be added to increase the resin viscosity without imparting thixotropy. Any high molecular weight material which is soluble in the monomer and which has no reactive functionality can be considered as a rheological additive. 
     THE COMPOSITION 
     The final composition contains no water or other solvent, but if one of the ingredients is a solid which is insoluble in the composition, it may be necessary to dissolve that ingredient in a small amount of solvent, using no more than necessary, in order to disperse it throughout the composition. Acetone is often useful for this purpose. The acetone then flashes off so that the final composition does not contain a solvent. Some initiators and some accelerators may require dissolution in solvent, but often the ingredients can be selected so as to avoid using a solvent. 
     The preferred method of preparing the composition is to first dissolve the oligomer, if one is used, in the monomer. To this mixture is added the rheological agent, if used, followed by a pre-mix of the initiators, the accelerator, and the inhibitor, if one is used. 
     The composition is then coated over a motor stator or other object or a cloth or fabric of glass, Dacron, or other material is impregnated with the composition. The composition is then irradiated with UV light (i.e., 200 to 410 μm) and is left to cure. While it is difficult to specify dosage, a satisfactory dosage is about 200 watts per linear inch for about 0.1 second up to about 5 minutes. 
    
    
     The following examples further illustrate this invention. 
     EXAMPLE 1 
     A number of compositions were prepared containing various thermal initiators. 
     
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Hexanediol diacrylate monomer                                             
                         100 pbw                                          
 Isopropyl ether of benzoin sold                                          
 by Stauffer Chemical Co. under the                                       
 trade designation &#34;V10&#34; 0.075 pbw                                        
Benzoic sulfimide        0.01 pbw                                         
Thermal initiator        0.075 pbw                                        
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     The following thermal initiators were used: 
     methyl ethyl ketone peroxide; 
     2,5-dimethyl-2,5-bis(hydroperoxy)hexane; 
     2,5-dimethyl-2,5-bis(t-butyl peroxy)hexane; 
     bis(t-butyl peroxy i-propyl)benzene; 
     azo bis isobutyronitrile; 
     cyano t-butyl azo cyclohexane; 
     hydrogen peroxide; 
     dicumyl peroxide; 
     lauroyl peroxide; 
     benzoyl peroxide; 
     cumene peroxide; 
     t-butyl hydroperoxide; 
     t-butyl perbenzoate; 
     di-t-butyl peroxide. 
     Sample of 10 grams of each of the fourteen compositions were placed in aluminum dishes 21/2 inches in diameter and were passed at 40 feet per minute under an RPC QC Processor ultraviolet light containing two bulbs, each producing 200 watts per linear inch. All of the compositions formed a polymeric skin after one pass and reacted completely with a vigorous exotherm after two passes. 
     When the photo-initiator was omitted from the composition, there was no reaction after thirteen passes. 
     EXAMPLE 2 
     Example 1 was repeated using various amine accelerators in the following composition. 
     
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Trimethylol propane triacrylate                                           
                        100     pbw                                       
t-butyl hydroperoxide   0.04    pbw                                       
&#34;V10&#34;                   0.2     pbw                                       
Amine accelerator       0.01    pbw                                       
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     After two passes at 100 feet per minute total reaction occurred with the following amine accelerators 
     methyl diethanolamine; 
     N,N-diethyl formamide; 
     N,N-dimethyl p-toluidine; 
     dipropylamine. 
     Two passes resulted in an incomplete reaction with the following amines, but additional passes would have completed the reaction. 
     dimethyl cyclohexylamine; 
     N,N-dimethyl formamide; 
     ethyl amino ethanol; 
     triethylamine. 
     The following non-aliphatic amines produced a surface skin polymerization but not a thermal reaction: 
     methyl vinyl pyrrolidone; 
     diethyl phenylene diamine; 
     dimethyl benzylamine; 
     N,N-dimethyl m-toluidine; 
     p-nitroaniline; 
     triphenylamine. 
     EXAMPLE 3 
     Part A 
     The 1:1 adduct of toluene diisocyanate and acrylic acid (30 to 70 wt.% in total) was dissolved in a mixture of tetraethylene glycol (3 parts) and ethyl methacrylate (1 part). To this was added &#34;V10&#34; (4 pbw), cumene hydroperoxide (2 pbw), N,N-dimethyl-p-toluidine (0.4 pbw), benzoic sulfimide (0.3 pbw), and hydroquinone (0.04 pbw). A 10 g cake in an aluminum dish was passed through a UV curing processor (2 lamps at 200 W/in) for approximately 10 sec. The reaction mixture then exothermed to give a hard, dry, transparent cake (about 100 mils thick) in about 2 min. 
     Part B 
     The reaction in Part A was repeated with 5 plies of glass cloth submerged in the resin. This also gave a hard cake under the same conditions. 
     Part C 
     Tetraethylene glycol (TEGDA, 7 pbw) and ethyl methacrylate (EMA, 3 pbw) were mixed with cumene hydroperoxide (2 pbw), N,N-dimethyl-p-toluidine (0.4 pbw), benzoic sulfimide (0.3 pbw), and hydroquinone (0.04 pbw). To this mixture was added V10 at the levels 4, 2, 1, 0.5, 0.3, and 0.1 pbw. Each of these combinations gave a dry cake, with a rubbery consistency, after 10 sec. irradiation and 2 min. exothermic reaction. 
     Part D 
     TEGDA (7 pbw), EMA (3 pbw) and V10 (0.1 pbw) were mixed with varying amounts of cumene hydroperoxide and accelerators. The thermal initiator combination was added at levels of 2.7, 2.0, 1.5, 1.0, 0.5, and 0.1 pbw. In all cases, they gave dry, rubbery cakes after irradiation and the subsequent reaction. 
     EXAMPLE 4 
     Example 1 was repeated using 100 pbw monomer, 1 pbw &#34;V10&#34;, 1 pbw di-tert-butyl perbenzoate, and 1 pbw methyl diethanolamine. Two 200 W/in lamps were used and the sample moved at 40 ft/min. The following gives the results with various monomers tested. 
     
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MONOMER                 REACTION                                          
______________________________________                                    
Trimethylol Propane Triacrylate                                           
                        Complete                                          
Pentaerythritol Triacrylate                                               
                        Complete                                          
Neopentyl Glycol Diacrylate                                               
                        Complete                                          
Hexanediol Diacrylate   Complete                                          
Triethylene Glycol Diacrylate                                             
                        Complete                                          
Tetraethylene Glycol Diacrylate                                           
                        Complete                                          
Hydroxy Ethyl Acrylate  Complete                                          
Ethyl Acrylate          Incomplete                                        
Butyl Acrylate          Incomplete                                        
2-Ethyl Hexyl Acrylate  Incomplete                                        
Phenoxy Ethyl Acrylate  Incomplete                                        
Methoxy Ethyl Acrylate  Incomplete                                        
Ethoxy Ethoxy Ethyl Acrylate                                              
                        Incomplete                                        
n-Hexyl Acrylate        Incomplete                                        
Ethoxy Ethyl Acrylate   Incomplete                                        
n-Lauryl Acrylate       Incomplete                                        
Trimethylol Propane Trimethacrylate                                       
                        Complete                                          
Triethylene Glycol Dimethacrylate                                         
                        Complete                                          
1,3 i-Butylene Dimethacrylate                                             
                        Complete                                          
Methacrylic Acid (Glacial)                                                
                        Complete                                          
Hydroxy Propyl Methacrylate                                               
                        Incomplete                                        
2-Hydroxy Ethyl Methacrylate                                              
                        Incomplete                                        
Methyl Methacrylate     No Reaction                                       
Glycidyl Methacrylate   No Reaction                                       
Ethyl Methacrylate      No Reaction                                       
Allyl Methacrylate      No Reaction                                       
Cyclohexyl Methacrylate No Reaction                                       
Vinyl Acetate           No Reaction                                       
Divinyl Benzene         No Reaction                                       
Vinyl Toluene           No Reaction                                       
Diallyl Phthalate       No Reaction                                       
Diallyl Isophthalate    No Reaction                                       
N-Vinyl 2-Pyrrolidone   No Reaction                                       
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     The fact that the monomers do not react by themselves does not indicate that they may not react when mixed with more reactive monomers. 
     EXAMPLE 5 
     Example 1 was repeated using different concentrations of initiators and accelerators in order to better define operative ranges. The following gives the results of testing three different compositions, each containing different proportions. 
     
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                            Concentrations                                
                Concentrations                                            
                            which do                                      
Material        which react not react                                     
______________________________________                                    
A.  Hexanediol diacrylate                                                 
                     100    parts  100  parts                             
    V10              0.02   part   0.01 part                              
    Benzoyl peroxide 0.1    part   0.05 part                              
    Triethylamine    0.01   part   0.005                                  
                                        part                              
    Benzoic sulfimide                                                     
                     0.01   part   0.005                                  
                                        part                              
B.  Hexane Diacrylate                                                     
                     100    parts  100  parts                             
    V10              0.02   part   0.01 part                              
    Di t-butyl peroxide                                                   
                     0.1    part   0.05 part                              
    Dimethyl c-Hexylamine                                                 
                     0.01   part   0.005                                  
                                        part                              
    Benzoic sulfimide                                                     
                     0.01   part   0.005                                  
                                        part                              
C.  Hexanediol diacrylate                                                 
                     100    parts  100  parts                             
    V10              0.02   part   0.01 part                              
    Cumene hydroperoxide                                                  
                     0.1    part   0.05 part                              
    Methyl diethanolamine                                                 
                     0.01   part   0.005                                  
                                        part                              
    Benzoic sulfimide                                                     
                     0.01   part   0.005                                  
                                        part                              
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     EXAMPLE 6 
     The following composition was prepared and cured as in Example 1. 
     
         ______________________________________                                    
Tetraethylene glycol diacrylate                                           
                         75 pbw                                           
Ethyl methacrylate       25                                               
Cumene hydroperoxide     2                                                
N,N-dimethyl-p-toluidine 0.4                                              
Benzoic sulfimide        0.3                                              
&#34;V10&#34;                    4.0                                              
Hydroquinone             0.04                                             
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     Samples of the composition were tested for electrical properties. The following are the results. 
     
         ______________________________________                                    
60 Hz electric strength on                                                
0.1 inch thick resin cake =                                               
                      400      V/mil                                      
                               at 25° C.                           
Dielectric constant at 25° C. =                                    
                      4.8                                                 
at 77° C. =    9.4                                                 
at 104° C. =   11.0                                                
Dissipation factor (100 tan δ) at 25° C. =                   
                      3.7%                                                
at 77° C. =    11.0%                                               
at 104° C. =   14.0%.                                              
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     These electrical values are acceptable for insulation purposes. 
     EXAMPLE 7 
     The following compositions were prepared using various photo-initiators. 
     
         ______________________________________                                    
Trimethylolpropane triacrylate                                            
                      100      pbw                                        
Cumene hydroperoxide  0.075                                               
N,N-dimethyl-p-toluidine                                                  
                      0.015                                               
Benzoic sulfimide                                                         
 (10% solution in acetone)                                                
                      0.01                                                
Hydroquinone          0.0015                                              
Photo-initiator       0.2                                                 
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     The compositions were irradiated as in Example 1. The following results were obtained: 
     
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Photo-Initiator        Comment                                            
__________________________________________________________________________
&#34;V10&#34;                  Complete reaction in 2 sec.                        
Benzoin butyl ether (sold by                                              
                       Complete reaction in 2 sec.                        
Stauffer Chemical Co. under                                               
the trade designation &#34;V30&#34;)                                              
Diethoxy acetophenone  Complete reaction in 4 sec.                        
Chlorothioxanthone/methyl                                                 
                       Complete reaction in 4 sec.                        
diethanolamine (1:1 ratio by wt.)                                         
A 1:1 weight ratio of Chloro-                                             
                       Complete reaction in 6 sec.                        
thioxanthone to &#34;Q43&#34;, a pentaer-                                         
ithrytol tetrakis (mercaptopro-                                           
pionate) sold by Cincinnati                                               
Milicron Co.                                                              
Benzophenone/Methyl    Complete reaction in 12 sec.                       
diethanolamine (1:1)                                                      
Chlorothioxanthone (saturated                                             
                       Complete reaction in 14 sec.                       
solution in acetone)                                                      
Benzophenone           Complete reaction in 20 sec.                       
Benzophenone/Mischler&#39;s                                                   
                       Complete reaction in 24 sec.                       
ketone (1:1)                                                              
Benzophenone/tetrahydrofuran (1:1)                                        
                       Complete reaction in 24 sec.                       
Mischler&#39;s ketone      Surface skin only                                  
Mischler&#39;s ketone/methyl-                                                 
diethanolamine (1:1)   Surface skin only                                  
__________________________________________________________________________
 
    
     EXAMPLE 8 
     10 gram quantities of different combinations of reactants were mixed in aluminum dishes (2.5 in dia.) and irradiated (2 lamps at 200 W/linear inch). The dishes were passed under the lamps at a speed of 40 ft/min, so that each pass was equivalent to about 2 seconds irradiation time. The materials used were 100 pbw hexanediol diacrylate (HDDA) by weight, 0.015 pbw &#34;V10&#34;, 0.08 pbw benzoyl peroxide (BP), 0.03 pbw N,N-dimethyl p-toluidine (NNDMT), and 0.03 pbw benzoic sulfimide (BS). 
     
         __________________________________________________________________________
Reaction Mixture     Comments                                             
__________________________________________________________________________
HDDA + &#34;V10&#34;         No reaction after 10 passes                          
HDDA + &#34;V10&#34; + BS    No reaction after 10 passes                          
HDDA + &#34;V10&#34; + BP + BS                                                    
                     No reaction after 10 passes                          
HDDA + &#34;V10&#34; + NNDMT + BS                                                 
                     Slight reaction after 10 passes                      
HDDA + &#34;V10&#34; + BP + NNDMT                                                 
                     Exothermic reaction after 8 passes                   
HDDA + &#34;V10&#34; + BP + NNDMT + BS                                            
                     Exothermic reaction after 5 passes.                  
__________________________________________________________________________
 
    
     The above experiments show that unless a photo-initiator, a thermal initiator, and an accelerator are all present, a complete reaction does not occur.