Light curing compositions for dental restoration
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light curing composition for dental restoration and, more particularly, to a light curing composition for dental restoration which shows high sensitivity with respect to light sources for emitting both visible light and ultraviolet light, and provides a polymerization-cured mass excelling in color stability and physical properties. 
     2. State of the Prior Art 
     As well known in the art, the so-called chemical curing type and light curing type of dental composite resins for restoration are used in dentistry to which the invention belongs. The chemical curing type of composite resins for filling is a pasty product composed mainly of a finely divided inorganic filler and a binder resin for bonding together filler particles. This pasty product is divided into two portions, one portion containing a peroxide and the other portion containing an amine. For use, these pasty portions are mixed together, and the resulting mixture is filled in a dental caries cavity. Thereafter, the binder-resin are polymerized and cured by the peroxide-amine redox reaction within a certain period of time. In view of manipulation, this type of resin has a disadvantage that its curing time is so certain that an operator should finish filling work while the paste still maintains its plasticity prior to curing. 
     With reference to the light curing type of composite resins for filling which makes use of a photo-polymerization catalyst in place of the peroxide-amine base catalyst used for chemical polymerization, the curing reaction does not proceed, unless they are exposed to light. For that reason, they have a number of advantages that their mainipulation and curing time is substantially controlled depending upon an operator&#39;s will so as to obtain the optimum results in respect of the treatment procedure and time to be applied. In dentistry, the light curing type of filling materials now enjoys increasing use. 
     Out of a number of compositions for the light curing type dental filling materials known in the art, there are often used visible light curing type of filling materials in view of the fact that they can securely be used in the oral mouth, in particular. For instance, such compositions are disclosed in British Pat. No. 1,408,265 as well as Japanese Patent Laid-Open Publication Nos. 57-187377, 53-62394, 57-54107, 57-77609 and 58-65704, which relate to the use of an alpha-diketone and a reducing agent, as disclosed in British Pat. No. 1,408,265 specification. In most cases, however, actually available products make use of camphor quinone.amine combinations. 
     The visible light curing type of filling materials cures in a wavelength region of 400-500 nm, but does not provide any satisfactory cured mass in a wavelength region of below 400 nm. They are also susceptible to the intensity of light, so that insufficient reactions tend to take place in a portion away from a light-emitting source, leaving a larger amount of the residual monomer behind. This often results in deteriorations in the color stability and physical properties from the clinical point-of-view. 
     Ultraviolet light curing type of dental compositions is known from Japanese Patent Publication No. 51-2235 and Japanese Patent Laid-Open Publication No. 53-82088, for instance. Ultraviolet light having a wavelength of 300 nm or lower cannot directly be used in the oral mouth, since they are harmful to the human body. As pointed our previously, their defect is a limited curing depth. 
     The aforesaid two types of filling compositions are designed to cure in their exclusive wavelength regions. Thus, any light curing type of polymerizable compositions is not still known in the art, which are capable of curing well in both ultraviolet and visible light regions. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a light curing composition for dental restoration of high sensitivity. 
     According to the present invention, it has been found that the coexistence of three constitutional components, i.e., two photo-polymerization initiators, a ketal base compound and a thioxanthone base compound, and one reducing agent produces a photo-polymerization initiation capability in both ultraviolet and visible light regions due to their synergistic effect. 
     Use of the ketal base compound alone or in combination with the reducing agent is effective for ultraviolet light of below 400 nm, but is substantially ineffective for visible light of 400 nm or higher in view of the photo-polymerization initiation capability. On the other hand, use of the thioxanthone base compound alone or in combination with the reducing agent produces little or no photo-polymerization initiation capability for visible light of 400 nm or higher, whereby any satisfactory cured mass is not obtained. Compositions comprising a mixture system of the ketal and thioxanthone base photo-polymerization initiators without using any reducing agent are found to show a slight photo-polymerization property, but their cured mass has insufficient strength and is practically useless, when used as the dental restorative materials. 
     It has now been found that the compositions of the present invention comprising a combination of a ketal base compound, a thioxanthone base compound and a reducing agent show preferred properties in dentistry, since they excel especially in the polymerization initiation capability for light of 300-500 nm and their cured mass excels in strength and hardness, and do also exhibit improved color stability which is an important factor in dentistry. 
     More specifically, the present invention provides a light curing composition for dental restoration characterized by comprising the following five constitutional components: 
     (a) a polymerizable compound containing at least one ethylenically unsaturated double-bond, 
     (b) at least one photo-polymerization initiator selected from ketal base compounds expressed in terms of the following general formula [1]: ##STR1## wherein X is H, Cl, an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms, 
     A is a six-membered aromatic group, and 
     R is an alkyl group having 1 to 10 carbon atoms, an aralkyl group having 7 to 9 carbon atoms or a --C n  H 2n  O) m  R&#39; in which n is an integer of 2 to 5, m is an integer of 1 to 5 and R&#39; is an alkyl group having 1 to 5 carbon atoms, 
     (c) at least one photo-polymerization initiator selected from thioxanthone base compounds expressed in terms of the following general formula [2]: ##STR2## wherein X indicates &gt;S, &gt;SO and &gt;SO 2 , R and R&#39; each represents a lower alkyl group, a substituted alkyl group, an alkoxy group, a halogen atom, a nitro group or a divalent unsaturated group forming a condensation ring structure, and m and n each denote 0 or an interger of 1 to 4, 
     (d) a reducing agent, and 
     (e) a filler. 
     The first constitutional components in the compositions of the present invention, i.e., the ethylenical compounds refer to those having in their chemical structure at least one ethylenically unsaturated double-bond, and taking on the chemical form of monomers, prepolymers (viz., dimers, trimers and other oligomers) or mixtures and copolymers thereof. 
     Concretely, the monomers having therein one ethylenically unsaturated double-bond include methyl, ethyl, isopropyl, hydroxyethyl, tetrahydrofurfuryl and glycidyl acrylates and methacrylates, and the monomers having therein two ethylenically unsaturated double-bonds include 2,2-bis(methacryloxyphenyl)propane, 2,2-bis[4-(2-hydroxy-3-methacryloxyphenyl)]propane, 2,2-bis(4-methacryloxyethoxyphenyl)propane, 2,2-bis(4-methacryloxydiethoxyphenyl)propane, 2,2-bis(4-methacryloxypropoxyphenyl)propane and acrylates thereof. The aliphatic monomers include ethylene glycol, diethylene glycol, triethylene glycol, butylene glycol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol and 1,6-hexanediol dimethacrylates and acrylates thereof. The monomers having therein three ethylenically unsaturated double-bonds include trimethylolpropane, trimethylolethane, pentaerythritol and trimethylolmetane trimethacrylates and acrylates thereof. The monomers having therein four ethylenically unsaturated double-bonds include pentaerythritol tetramethacrylate and pentaerythritoltetracrylate as well as the urethane base monomers expressed in terms of the following structural formulae (3) and (4): ##STR3## Other monomers include 4-methacryloxyethyl trimellitic acid and its anhydrides. These acrylates and methacryates may be used alone or in combination. 
     Preferably, these compounds having an ethylenically unsaturated double-bond or bonds should be used in an amount of 90-10 weight % with respect to the filler. When the amount of the compounds having an ethylenically unsaturated double-bond or bonds departs from the aforesaid range, any satisfactory filling material is not formed. 
     The ketal base compounds used as the second constitutional component and expressed in terms of the general formula [1] include, in addition to benzil dimethyl ketal having the following formula: ##STR4## benzil diethyl ketal, benzil dipropyl ketal, benzil-di(β-phenylethyl)ketal, benzil-di(2-methoxyethyl)ketal, benzil-di(2-ethoxyethyl)ketal, benzil-di(2-methoxyethoxyethyl)ketal, benzil-di(2-ethoxyethoxyethyl)ketal, 4,4&#39;-dimethylbenzil-dimethyl ketal, 2,2&#39;-dimethoxybenzil-diethyl ketal, 4,4&#39;-dichlorobenzil-diethyl ketal, 4,4&#39;-dichlorobenzil-dipropyl ketal and so on. Among others, particular preference is given to benzil dimethyl ketal, benzil diethyl ketal, benzil-di(2-methoxyethyl)ketal and 4,4&#39;-dimethylbenzil-dimethyl ketal. 
     The thioxanthone base compounds used as the third constitutional component in the present invention include thioxanthone, 2-chlorothioxanthone, 2-isopropylthioxanthone, 2-nitrothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, 2-chloro-7-trifluoromethylthioxanthone, thioxanthone-10,10-dioxide, thioxanthone-10-oxide. Among the rest, preference is given to thixanthone, 2-chlorothioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone and 2,4-diisopropylthioxanthone. These photo-polymerization initiators may be used alone or in combination thereof. 
     Referring to the amounts of the aforesaid ketal and thioxanthone base photo-polymerization initiators added, it is preferred that they be added in an amount of 0.01 to 5 weight % relative to the compound having an ethylenically unsaturated double-bond or bonds. In ranges departing from those as mentioned just above, the resulting curing property and color stability are practically unsuitable for use in dentistry. 
     As the reducing agents that are the fourth costitutional component in the present invention, use is made of compounds which are capable of reducing photosensitizers, when they are excited, but are incapable of reducing them, when they are not excited by active energy beams. The reducing agents may be primary, secondary or tertiary amines. In the amine ##STR5## none of R, R&#39; and R&#34; may be a hydrogen atom, or one or two of R, R&#39; and R&#34; may be a hydrogen atom. One or more groups of R, R&#39; and R&#34; may be different or identical hydrocarbon groups, which may be alkyl, cycloalkyl, hydroxyalkyl or aralkyl groups, for instance. Preferred R, R&#39; and R&#34; groups are C 1  -C 10  alkyl groups. 
     Suitable examples of the reducing agents wherein one or more groups represented by R, R&#39; and R&#34; are hydrocarbons include propylamine, n-butylamine, pentylamine, hexylamine, dimethylamine, diethylamine, dipropylamine, di-n-butylamine, dipentylamine, trimetylamine, triethylamine, tripropylamine, tri-n-butylamine, tripentylamine, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, triethanolamine, dimethylaminoethanol and long-chain aliphatic amines. 
     Examples of the reducing agents containing an aromatic group include N,N&#39;-dimethylaniline, N-methyldiphenylamine, 2-dimethylaminobenzoic acid ethyl ester, 4-dimethylaminobenzoic acid ethyl ester, 4-dimethylaminobenzoic acid methyl ester, 4-dimethylaminobenzoic acid butyl ester, 4-dimethylaminobenzoic acid 2-ethylhexyl ester and 4-dimethylaminobenzoic acid isoamyl ester. 
     Use may be made of a diamine having the structure of ##STR6## wherein n is an integer of 2 or higher, and different or identical groups R, R&#39;, R&#34; and R&#39;&#34; are a hydrogen atom or hydrocarbon groups, particularly alkyl groups. This type of reducing agent may be exemplified by ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine or hexamethylenediamine or N-hydrocarbon derivatives, especially N-alkyl derivatives. 
     Examples of the reducing agents in which an element N forms a part of ring include, for instance, piperidine and N-hydrocarbon derivatives thereof. 
     Other reducing agents which may be used in the present invention include triaryl amines, allyl thiourea, aromatic sulfinates, 5-alkyl or 5-aryl-barbituric acid and so on. 
     Among these reducing agents, preference is given to dimethylaminoethyl methacrylate, triethanolamine, 4-dimethylaminobenzoic acid methyl ester and 4-methylaminobenzoic acid ethyl ester. 
     The concentration of these reducing agents is preferably 0.01 to 5  weight % based on the ethylenically unsaturated substances in view of dental color stability and curing property. 
     In addition to the aforesaid constitutional components, if required, the compositions of the present invention may contain polymerization inhibitors, ultraviolet absorbers and organic peroxides which are usually used. 
     The fifth constitutional filler component may be inorganic and/or organic fillers. Suitable examples of fillers include inorganic fillers such as quartz, alumima, glass, kaolin, talc, calcium carbonate, barium aluminosilicate glass, titanium oxide, borosilicate glass, and colloidal silica and the so-called organic composite filler in which an inorganic filler is coated with an organic polymer and organic fillers such as polymethyl acrylate, polymethyl methacrylate, polyethyl methacrylate, copolymers of methyl methacrylate with ethyl methacrylate, crosslinked type polymethyl methacrylate and copolymers of ethylene with vinyl acetate. These polymer powders may be used in the form of mixtures with the aforesaid inorganic powders. 
     It is preferred that, prior to mixing the inorganic filler with the binder resin, that filler is treated on its surface with a coupling agent capable of reacting with both the filler and the binder resin. The coupling agents used may include a silane coupling agent, a titanate coupling agent, an aluminate coupling agent and so on. Alternatively, the inorganic filler may be grafted on the surface for bonding to the binder resin. 
     The silane coupling agents used to this end include γ-methacryloxypropyl trimethoxysilane, vinyltrichlorosilane, vinyl-tris(β-methoxyethoxy)silane, γ-methacryloxypropylmethyl dimethoxysilane, γ-glycidoxypropyl trimethoxysilane, γ-chloropropyl trimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl trimethoxysilane, trimethylchlorosilane, dimethyldichlorosilane, hexamethyldisilane, γ-aminopropyl triethoxysilane, N-β-(aminoethoxy)-γ-aminopropyl trimethoxysilane, -urenoidopropyltrimethoxysilane and so on. 
     In the present invention, any method for surface treatment with these coupling agents may be used. The amount of said surface treatment agents used varies depending upon the nature and state required, and is not generally determined. Generally, however, said surface treatment agents may be used in an amount ranging from 0.1 to 20 weight %, preferably 1 to 10 weight %. 
     The active energy beams used in the present invention may be visible light or ultraviolet light, or may contain their spectra both visible and ultraviolet light. A preferred wavelength ranges from 240 nm to 600 nm. The light sources applicable to the compositions of the present invention include carbon arc, mercury lamps, xenon lamps, metal halide lamps, fluorescent lamps, tungsten lamps and argon ion laser. 
     The present invention will now be explained in further detail with reference to the following non-restrictive examples and comparison examples. 
    
    
     EXAMPLE 1 
     Seventy (70) grams of 2,2-bis[4-(2-hydroxy-3-methacryloxy)phenyl]propane, 30 grams of triethylene glycol dimethacrylate, 0.5 grams of benzil dimethyl ketal, 0.5 grams of thioxanthone and 0.5 grams of dimethylaminoethyl methacrylate were roll-mixed at room temperature with 100 grams of finely divided silica treated on the surface with 3 grams of α-methacryloxypropyl trimethoxysilane to prepare a composition. The paste was then filled in a Teflon plate formed therein with a round hole having a diameter of 3 mm and a thickness of 3 mm, and the assembly was exposed, from 1 mm above, to visible light (Trade Name: Luxor made by ICI) to determine the color stability and compressive strength of the cured mass. Examination was made of the rate of penetration of an 1 kg-loaded needle into the surface of the test piece opposite to that thereof to be exposed to visible light to express the curing time in terms of the shortest visible light-exposing time in which the rate of penetration assumed zero. In the tables to be given below, the bracketed figures are the curing time measured with ultraviolet light (Trade Name: Permacure-UC-1 made by GC) in the same manner. A stainless steel mold was also provided therethrough with a round hole of 4 mm in diameter and 10 mm in thickness, in which said paste was filled, followed by covering the surface thereof with a cellophane paper. Thereafter, the assembly was exposed to visible light (Trade Name: Luxor made by ICI) for 45 seconds. The polymer was then taken out of the round hole to remove unreacted matters, and was measured in respect of its length for curing depth. The results are set forth in the tables. 
     EXAMPLES 2-11 AND COMPARISON EXAMPLES 1-10 
     In accordance with Example 1, examinations were made of the curing time, color stability, compressive strength and curing depth of various compositions, wherein the ketal base compounds, thioxanthone base compounds and reducing agents set forth in the tables were used in the amounts specified therein in place of those described in Example 1. In Comparison Examples 1, 2 and 3, experiments were each carried out without using the ketal base compound, thioxanthone base compound and reducing agent, respectively. In Comparison Example 10, use was made of camphor quinone which enjoys wide use as commercially available products. The results are summarized in the tables. 
     EXAMPLES 12, 13 AND 14 
     In accordance with Example 1, examinations were made of the curing time, color stability, compressive strength and curing depth of various compositions, wherein 70 grams of an aliphatic urethane dimethacrylate and 30 grams of butanediol dimethacrylate were used as the monomer components in the composition of Example 1, and the ketal and thioxanthone base compounds and reducing agent set forth in the tables were used in the amounts specified therein. The results are summarized in the tables. 
     
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       Example 1                                                          
                Example 2                                                 
                         Example 3   Example 4  Example                   
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                                                5                         
Ketal Base                                                                
       Benzil Dimethyl                                                    
                Benzil Dimethyl                                           
                         Benzil Dimethyl                                  
                                     Benzil Dimethyl                      
                                                Benzil Dimethyl           
Compound                                                                  
       Ketal    Ketal    Ketal       Ketal      Ketal                     
Amount (g)                                                                
       0.5      0.5      0.5         0.5        0.5                       
Thioxanthone                                                              
       Thioxanthone                                                       
                2,4-dimethyl-                                             
                         2,4-diethyl-                                     
                                     2,4-diisopropyl-                     
                                                2-chloro-                 
Base            thioxanthone                                              
                         thioxanthone                                     
                                     thioxanthone                         
                                                thioxanthone              
Compound                                                                  
Amount (g)                                                                
       0.5      0.5      0.5         0.5        0.5                       
Reducing                                                                  
       Dimethylamino-                                                     
                Dimethylamino-                                            
                         Dimethylamino-                                   
                                     Dimethylamino-                       
                                                Dimethylamino-            
Agent  ethylmethacrylate                                                  
                ethylmethacrylate                                         
                         ethylmethacrylate                                
                                     ethylmethacrylate                    
                                                ethylmethacrylate         
Amount (g)                                                                
       0.5      0.5      0.5         0.5        0.5                       
Curing Time                                                               
Visible Light                                                             
       35&#34;      35&#34;      35&#34;         40&#34;        30&#34;                       
(min. sec.)                                                               
Ultraviolet                                                               
       (1&#39; 35&#34;) (1&#39; 40&#34;) (1&#39; 40&#34;)    (1&#39; 50&#34;)   (1&#39; 30&#34;)                  
Light                                                                     
(min. sec.)                                                               
Compressive                                                               
       3777     3663     3750        3562       4003                      
Strength                                                                  
(kg/cm.sup.2)                                                             
Curing Depth                                                              
        4.13     4.09     4.20        3.58       5.57                     
(mm)                                                                      
Color  Colorless                                                          
                Colorless                                                 
                         Colorless   Colorless  Colorless                 
Stability                                                                 
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       Example 6                                                          
                Example 7                                                 
                         Example 8   Example 9  Example                   
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                                                10                        
Ketal Base                                                                
       Benzil Dimethyl                                                    
                Benzil Dimethyl                                           
                         Benzil Dimethyl                                  
                                     Benzil Diethyl                       
                                                Benzil-di(2-              
Compound                                                                  
       Ketal    Ketal    Ketal       Ketal      Methoxyethyl)Ketal        
Amount (g)                                                                
       0.5      0.5      0.5         0.5        0.5                       
Thioxanthone                                                              
       2-chloro-                                                          
                2-chloro-                                                 
                         2-chloro-   2-chloro-  2-chloro-                 
Base   thioxanthone                                                       
                thioxanthone                                              
                         thioxanthone                                     
                                     thioxanthone                         
                                                thioxanthone              
Compound                                                                  
Amount (g)                                                                
       0.5      0.2      0.2         0.2        0.2                       
Reducing                                                                  
       4-Dimethylamino-                                                   
                Triethanolamine                                           
                         4-Dimethylamino-                                 
                                     4-Dimethylamino-                     
                                                4-Dimethylamino-          
Agent  benzoic Acid Ethyl                                                 
                         benzoic Acid Methyl                              
                                     benzoic Acid Ethyl                   
                                                benzoic Acid Ethyl        
       Ester             ester       ester      ester                     
Amount (g)                                                                
       0.5      0.5      0.5         0.5        0.5                       
Curing Time                                                               
Visible Light                                                             
       30&#34;      30&#34;      35&#34;         30&#34;        35&#34;                       
(min. sec.)                                                               
Ultraviolet                                                               
       (1&#39; 30&#34;) (1&#39; 30&#34;) (1&#39; 40&#34;)    (1&#39; 30&#34;)   (1&#39; 35&#34;)                  
Light                                                                     
(min. sec.)                                                               
Compressive                                                               
       4029     3899     3612        3927       3700                      
Strength                                                                  
(kg/cm.sup.2)                                                             
Curing Depth                                                              
        5.49     5.50     4.23        5.60       4.57                     
(mm)                                                                      
Color  Colorless                                                          
                Colorless                                                 
                         Colorless   Colorless  Colorless                 
Stability                                                                 
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       Example 11  Example 12   Example 13    Example 14                  
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Ketal Base                                                                
       4,4&#39;-dimethylbenzil-                                               
                   Benzil Dimethyl                                        
                                Benzil Dimethyl                           
                                              Benzil                      
                                                    Benzil                
Compound                                                                  
       dimethyl Ketal                                                     
                   Ketal        Ketal         Dimethyl                    
                                                    Diethyl               
                                              Ketal Ketal                 
Amount (g)                                                                
       0.5         0.5          0.5           0.25  0.25                  
Thioxanthone                                                              
       2-chloro-   2-chloro-    2-chloro-                                 
                                       Thio-  2-chloro-                   
Base   thioxanthone                                                       
                   thioxanthone thioxanthone                              
                                       xanthone                           
                                              thioxanthone                
Compound                                                                  
Amount (g)                                                                
       0.2         0.2          0.1    0.1    0.2                         
Reducing                                                                  
       4-Dimethylamino-                                                   
                   4-Dimethylamino-                                       
                                4-Dimethylamino-                          
                                               Dimethylamino-             
Agent  benzoic Acid Ethyl                                                 
                   benzoic Acid Ethyl                                     
                                benzoic Acid Ethyl                        
                                              benzoic Acid Ethyl          
       ester       ester        Ester         Ester                       
Amount (g)                                                                
       0.5         0.5          0.5           0.5                         
Curing Time                                                               
Visible Light                                                             
       35&#34;         25&#34;          25&#34;           25&#34;                         
(min. sec.)                                                               
Ultraviolet                                                               
       (1&#39; 40&#34;)    (1&#39; 20&#34;)     (1&#39; 15&#34;)      (1&#39; 15&#34;)                    
Light  (min. sec.)                                                        
Compressive                                                               
       3666        4078         4120          4119                        
Strength                                                                  
(kg/cm.sup.2)                                                             
Curing Depth                                                              
        4.47        5.93         5.88          5.91                       
(mm)                                                                      
Color  Colorless   Colorless    Colorless     Colorless                   
Stability                                                                 
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       Comparative                                                        
                  Comparative                                             
                             Comparative                                  
                                      Comparative                         
                                                Comparative               
       Example 1  Example 2  Example 3                                    
                                      Example 4 Example                   
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                                                5                         
Ketal Base                                                                
       Benzil Dimethyl                                                    
                  --         Benzil Dimethyl                              
                                      Benzil Dimethyl                     
                                                Benzil Dimethyl           
Compound                                                                  
       Ketal                 Ketal    Ketal     Ketal                     
Amount (g)                                                                
       0.5                   0.5       0.005      6                       
Thioxanthone                                                              
       --         2-chloro-  2-chloro-                                    
                                      2-chloro- 2-chloro-                 
Base              thioxanthone                                            
                             thioxanthone                                 
                                      thioxanthone                        
                                                thioxanthone              
Compound                                                                  
Amount (g)        0.2        0.2      0.2       0.2                       
Reducing                                                                  
       Dimethylaminoethyl-                                                
                  Dimethylaminoethyl-                                     
                             --       4-Dimethylamino-                    
                                                4-Dimethylamino-          
Agent  methacrylate                                                       
                  methacrylate        benzoic Acid Ethyl                  
                                                benzoic Acid Ethyl        
                                      Ester     Ester                     
Amount (g)                                                                
       0.5        0.5                 0.5       0.5                       
Curing Time                                                               
Visible Light                                                             
       Not cured  &gt;2&#39;  00&#34;   2&#39; 00&#34;   &gt;2&#39; 00&#34;   35&#34;                       
(min. sec.)                                                               
Ultraviolet                                                               
       (1&#39; 50&#34;)   (&gt;10&#39; 00&#34;) (4&#39; 45&#34;) (&gt;10&#39; 00&#34;)                          
                                                (1&#39; 45&#34;)                  
Light                                                                     
(min. sec.)                                                               
Compressive                                                               
       --         --         --       --        1821                      
Strength                                                                  
(kg/cm.sup.2)                                                             
Curing Depth                                                              
       --         --         --       --         3.30                     
(mm)                                                                      
Color  --         --         --       --        Colorless                 
Stability                                                                 
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       Comparative                                                        
                 Comparative                                              
                            Comparative                                   
                                      Comparative                         
                                                Comparative               
       Example 6 Example 7  Example 8 Example 9 Example                   
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                                                10                        
Ketal Base                                                                
       Benzil Dimethyl                                                    
                 Benzil Dimethyl                                          
                            Benzil Dimethyl                               
                                      Benzil Dimethyl                     
                                                Camphorquinone            
Compound                                                                  
       Ketal     Ketal      Ketal     Ketal                               
Amount (g)                                                                
       0.5       0.5        0.5       0.5       0.5                       
Thioxanthone                                                              
       2-chloro- 2-chloro-  2-chloro- 2-chloro- --                        
Base   thioxanthone                                                       
                 thioxanthone                                             
                            thioxanthone                                  
                                      thioxanthone                        
Compound                                                                  
Amount (g)                                                                
        0.005      6        0.2       0.2                                 
Reducing                                                                  
       4-Dimethylamino-                                                   
                 4-Dimethylamino-                                         
                            4-Dimethylamino-                              
                                      4-Dimethylamino-                    
                                                Hexylamine                
Agent  benzoic Acid Ethyl                                                 
                 benzoic Acid Ethyl                                       
                            benzoic Acid Ethyl                            
                                      benzoic Acid Ethyl                  
       Ester     Ester      Ester     Ester                               
Amount (g)                                                                
       0.5       0.5         0.005      6       0.5                       
Curing Time                                                               
Visible Light                                                             
       Not cured 35&#34;        &gt;2&#39; 00&#34;   35&#34;       45&#34;                       
(min. sec.)                                                               
Ultraviolet                                                               
       (1&#39; 50&#34;)  (1&#39; 40&#34;)   (4&#39; 45&#34;)  (1&#39; 45&#34;)  (Not cured)               
Light                                                                     
(min. sec.)                                                               
Compressive                                                               
       --        1359       --        2907      3401                      
Strength                                                                  
(kg/cm.sup.2)                                                             
Curing Depth                                                              
       --         4.28      --         4.77      3.08                     
(mm)                                                                      
Color  --        Brown      --        Yellow    Yellow                    
Stability                                                                 
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     As appreciated from the results of Examples 1 to 14, the compositions according to the present invention show more improved curing property with respect to the light source for both visible light and ultraviolet light over the prior art composition (Comparison Example 10), and are found to be cured by short-period irradiation and have a large curing depth. Furthermore, the cured masses obtained from the invented compositions show no sign of coloring and are substantially colorless, and thus have more improved color stability over the prior art cured mass (Comparison Example 10). The cured masses according to the present invention have also high strength. 
     The light curing compositions for dental restoration according to the present invention provide cured masses having improved physical properties by light irradiation in a short period of time, even though the filling material is large in thickness. 
     With the restorative materials obtained from the compositions of the present invention, clinically satisfactory results are obtained even after the lapse of two years in the oral mouth. In other words, they substantially show no sign of discoloration, wearing and breaking, unlike the conventional products. Thus, the restorative materials obtained from the compositions according to the present invention can satisafctorily be used for the dental purposes including dental composite resins for filling, synthetic resins for crown, resins for inlay, resins for jacket crown, resin teeth, acrylic denture base resins, repaired resins, acrylic resins for denture rebase, resins for impression tray and orthodontic resins.