Coating composition
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention concerns a coating composition with excellent abrasion resistance suitable to the surface coating of a substrate such as of metals, fiber-reinforced plastics (hereinafter simply referred to as FRP) and other plastics. 
     2. Description of the Prior Art 
     As metals, FRP and other plastic materials have been used more and more in electric and electronic applications in recent years, an improvement is demanded for the surface durability of these substrate materials such as abrasion resistance and scratch resistance. 
     Various methods of surface treatment through abrasion-resistant coating have been proposed for improving the durability of metal materials and wooden floor materials, for instance, as described below: 
     (1) a composition comprising from 0.5 to 30% by weight of mineral substances mainly composed of aluminum oxide and from 1 to 50% by weight of fiberous substances incorporated through dispersion to an acid curing type amino alkyd resin or moisture-curing type polyurethane resin solid (Japanese Patent Publication No. 35487/1976), 
     (2) a composition comprising from 5 to 7.5 parts by weight of α-alumina and from 2.5 to 5.0 parts by weight of silicon carbide blended with 100 parts by weight of a urethane resin (Japanese Patent Publication No. 33475/1980), 
     (3) a coating composition comprising highly hard ceramics incorporated and dispersed into an abrasion-resistant thermoplastic resin (Japanese Patent Laid-Open No. 4930/1979), etc. 
     However, any of the compositions (1) and (2) is used for improving the durability of floor materials and it is difficult to provide a high surface durability demanded for metal, FRP, plastics in electric and electronic applications. In addition, the compositions have only an insufficient adhesion property for these materials. Further, the abrasin-resistant thermoplastic resin in the above composition (3), for example, nylon, polyacetal or teflon has an insufficient adhesion property for metal, FRP, plastics and can not provide a surface coating treatment of high abrasin resistance and scratch resistance. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a coating composition capable of forming a coating film having an excellent hardness, abrasion resistance and scratch resistance, as well as being excellent in various properties such as smoothness, adhesion property and chemical-resistance. 
     The foregoing object can be attained by a coating composition excellent in abrasion resistance comprising spherical α-alumina particles, a binder and a solvent, in which said spherical α-alumina particles are contained in an amount of from 35 to 85% by weight of the solid content in the coating composition. 
     DETAILED DESCRIPTION OF THE INVENTION 
     There are no particular restrictions for the starting material and the production process of the spherical α-alumina particles. For instance, those spherical α-alumina particles prepared directly by using γ-alumina as an intermediate product and those spherical particles prepared by pulverizing fine sintered products of α-alumina and re-sintering them can be used preferably. It is necessary in this invention to always use spherical α-alumina particles since the use of non-spherical α-alumina in the coating composition may significantly reduce the abrasion resistance of the resulted coating films. 
     A preferred amount of blending the spherical α-alumina particles is usually from 35 to 85% by weight and, preferably, from 50 to 80% by weight to the solid content of the coating composition. If the blending amount is less than 35% by weight, the abrasion resistance and the scratch resistance are extremely reduced. While on the other hand, if the blending amount exceeds 85% by weight, the coating processability is decreased and the adhesion property to each of the substrates is undesirably reduced. 
     Referring then to the particle size of the spherical α-alumina particles employed, the average particle size within a range between 5-50 μm and, preferably, 5-30 μm shows the most excellent abrasion resistance. If the particle size exceeds 50 μm, the particles are liable to be caught and detached from the resin layer by the effect of the acting frictional member in the course of the sliding movement. On the other hand, if the average particle size is less than 5 μm, they tend to behave together with the binder in which the properties of the binder resin becomes significant tending to eliminate the spherical effect. 
     The binder used as the second component in this invention can include epoxy resin, acrylic resin, urethane resin, fluorine resin, silicone resin and alkyd resin. Among all, the use of combination of epoxy resin or hydroxyl radical containing acrylic resin and curing agents capable of reaction therewith, or the use of thermoplastic acrylic resin is preferred. 
     The epoxy resin used in this invention may be a bisphenol type epoxy resin with 450 or more of epoxy equivalent, which may of course be commercially available ones. If the epoxy equivalent is less than 450, adhesion property to each of the substrates is particularly reduced. In addition, the abrasion resistance and scratch resistance or the like of the resultant coating film are significantly reduced. 
     The hydroxyl radical containing acrylic resin usable in this invention may be such an acrylic resin having a glass transition temperature of higher than 30° C. and a hydroxyl value of from 20 to 120. If the glass transition temperature of the acrylic resin is less than 30° C., the abrasion resistance, the scratch resistance and the like of the obtained coating film are significantly reduced, and the stain resistance and weatherability are also reduced. Further, if the hydroxyl value of the acrylic resin is less than 20, the solvent resistance, surface hardness and the abrasion resistance of the obtained coating film are reduced. While on the other hand, if the hydroxyl value of the acrylic resin exceeds 120, the processability for the coating, adhesion property and weatherability are reduced. 
     The acrylic resin as described above is prepared by polymerizing a monomer mixture comprising an essential component of hydroxyl radical containing (metha)acrylate, for example, 2-hydroxyethyl (metha)acrylate, 2-hydroxypropyl (metha)acrylate and (metha)acrylic acid alkyl ester and, optionally, other vinyl monomer copolymerizable with the above-mentioned hydroxyl radical containing acrylic monomer, for example, methacrylic acid, acrylic acid, styrene, vinyl toluene, vinyl acetate, vinyl propionate and acrylonitrile, in the usual manner. 
     There is no particular restriction for the curing agent used in combination with the epoxy resin and the hydroxy radical containing acrylic resin as described above and the curing agent can properly be selected from known curing agents. They can include, for example, amino resins represented by methyl etherfied melamine resin, butyl etherfied melamine resin, butyl etherfied benzoguanamine resin and butyl etherfied urea resin; polyisocyanate compounds or resins having at least two isocyanate groups in one molecule; compounds or resins having at least two acid anhydride groups in one molecule; silicone resin having at least two alkoxy silane groups in one molecule; and the like. Among all, amino resins and polyisocyanate compounds or resins provide a significant effect as the curing agent for the abrasion resistant coating for metal and FRP or plastics used in electrical and electronic applications. By properly determining the blending amount of the curing agent in view of the amount of the reactive groups for curing depending on the combination of the resin and the curing agent, the properties of the coating film can optionally be controlled. Usually, the curing agent is blended in such a ratio that the amount of the reactive groups in the curing agent is equivalent or less to the amount of the reactive groups in the epoxy resin and the hydroxyl radical containing acrylic resin. 
     Thermoplastic acrylic resins having a glass transition temperature of higher than 40° C. and a weight average molecular weight of greater than 30,000 can be used as the acrylic resin in this invention. The thermoplastic acrylic resin is produced by polymerizing a monomer mixture comprising a methacrylic acid alkyl ester and/or acrylic acid alkyl ester as the essential ingredient and one or more of other vinyl monomers copolymerizable with the (metha)acrylic acid alkyl ester, for example, styrene, vinyl toluene, vinyl acetate, (metha)acrylic acid, itaconic acid, 2-hydroxy ethyl (metha)acrylate, 2-hydroxyl propyl (metha)acrylate in a usual manner by using a radical polymerization initiation catalyst. If the glass transition temperature of the thermoplastic acrylic resin is lower than 40° C., not only the abrasion resistance and the scratch resistance of the thus obtained coating film are significantly reduced but also the stain resistance and weatherability are also impaired. While on the other hand, if the thermoplastic acrylic resin with the molecular weight of less than 30,000 is used, the physical strength of the obtained coating film is significantly reduced undesirably. 
     There is no particular restriction for the solvents used in this invention so long as the coated substrates are not degraded with the solvent. For instance, they include hydrocarbons such as toluene and xylene; esters such as ethyl acetate and butyl acetate; ketones such as methyl ethyl ketone and methyl isobutyl kentone and alcohols such as ethanol, isobutanol and n-butanol. 
     In using the coating composition according to this invention, the binder, spherical α-alumina, solvent and, if required, the curing agent as described above are blended each in a predetermined amount by a usual manner, which is coated by means of spray coating, roll coater coating or the like and then dried to obtain a strong coating film. 
     Upon blending the composition, additives, dyes, pigments or the likes usually employed may be added within such a range as not impairing the effect of this invention. 
    
    
     EXAMPLES 1-3 
     As shown in Table 1, disphenol A type epoxy resins of different epoxy equivalents were dissolved in a mixed solvent of toluene and cellosolve acetate to prepare epoxy resin solutions. Then, after mixing to dissolve Yuban® 22R (registered trade mark; manufactured by Mitsui Toatsu Kagaku K.K.), which is a butyl etherfied melamine resin, into each of the epoxy resin solutions in a solid ratio of epoxy resin/melamine resin=60/40 (weight ratio), spherical α-alumina particles of 10 μm average particle size were added by 70% by weight converted as the solid component and stirred at a high speed to obtain coating materials (I), (II) and (III). 
     Each of the thus obtained coating materials was coated on a PBT resin molding product reinforced with 10% carbon fiber (manufactured by Mitsubishi Rayon K.K.) in a thickness of 40 μm and then baked at 130° C. for 40 min. Evaluations of the performance of the coating films on the coated articles obtained by this method are shown in Table 1. As the result, it can be seen that the coating materials according to this invention show extremely preferred abrasion resistance and scratched resistance. 
     
                       TABLE 1                                                     
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Coating material                                                          
            (I)      (II)      (III)  (IV)                                
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Epoxy equivalent                                                          
            470      920       2700   350                                 
of epoxy resin                                                            
α-alumina content                                                   
             50       70       80      70                                 
(% by weight)                                                             
Pencil hardness                                                           
            8H       more      more   3H                                  
                     than 9H   than 9H                                    
Abrasion resistance                                                       
             70       65       60     130                                 
(mg) (note 1)                                                             
Adhesion property                                                         
            100      100       95      60                                 
(note 2)                                                                  
Scratch resistance                                                        
            no abnor-                                                     
                     no abnor- no abnor-                                  
                                      slightly                            
(note 3)    mality   mality    mality scratched                           
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 (note 1): A taper type rotational abrasion tester and CS17 as a frictiona
 member were used and the abrasion resistance was determined as the amount
 after the tester was rotated for 2000 cycles under the speed of 1000     
 cycles per min and 1000 g of load.                                       
 (note 2): Cured film was scratched with a cutter knife at 2 mm interval t
 make 100 rectangular area and the tacky polyester tape was bonded and    
 thereafter peeled from the surface. The adhesion property was indicated b
 the number of the rectangles remained on the coated plate.               
 (note 3): The scratch resistance was defined based on the state of the   
 coated surface when rubbed 10 times with steel wools.                    
 
    
     COMPARATIVE EXAMPLE 1 
     For the comparison, paint was formulated under the quite similar conditions as in Example 1 except for using an epoxy resin having a epoxy equivalent of 350 as shown in Table 1 to obtain a coating material (IV). A coated plate was prepared by using the coating material (IV) under the same conditions as those in Example 1 and the performance evaluation was carried out. The result is shown in Table 1. 
     EXAMPLES 4-6 
     Monomer compositions as shown in Table 2 were polymerized by using azobis isobutylonitrile as the polymerization initiator at 80° C. to obtain toluene solutions of acrylic resin with different glass transition temperatures and hydroxyl values. After adding Takenate D-110N (manufactured by Takeda Yakuhin Kogyo K.K.), which is the polyisocyanate compound, into each of the acrylic resin solutions such that the polyisocyanate were in equivalent with the hydroxy equivalent in the acrylic resins, spherical α-alumina particles of 10 μm average particle size were added by 50%, 70% and 80% by weight converted as the solid component and stirred at a high speed to obtain coating materials (V), (VI) and (VII) respectively. 
     Each of the coating materials thus obtained was spray-coated on a sand blasted steel plate to a thickness of 30 μm and dried at 60° C. for 60 min. The results of the performance evaluation for the coating films of the thus coated plate obtained in this manner are shown in Table 2. It can be seen from the result, that the abrasion resistant coating materials according to this invention show extremely excellent abrasion resistance and scratch resistance. 
     
                       TABLE 2                                                     
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                       Comparative                                        
         Example       Example                                            
           4       5       6     2      3                                 
Coating material                                                          
           (V)     (VI)    (VII) (VIII) (IX)                              
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Monomer compo-                                                            
sition for                                                                
polymerization                                                            
(% by weight)                                                             
Methyl methacry-                                                          
           30      25      25    --     30                                
late                                                                      
Styrene    30      30      25    --     30                                
Butyl methacry-                                                           
           30      30      30    80     36                                
late                                                                      
2-hydroxylethyl                                                           
           10      15      20    20      4                                
methacrylate                                                              
Property of                                                               
acrylic resin                                                             
Glass transition                                                          
           70      66      64    27     66                                
temperature (°C.)                                                  
Hydroxyl-value                                                            
           43      64      86    86     17                                
α-alumina content                                                   
           50      70      80    80     70                                
(% by weight)                                                             
Abrasion resis-                                                           
           80      70      82    115    120                               
tance (mg) (1)                                                            
Adhesion   100     100     95    100    100                               
property (2)                                                              
Scratch    no      no      no    slightly                                 
                                        slightly                          
resistance (3)                                                            
           abnor-  abnor-  abnor-                                         
                                 scratched                                
                                        scratched                         
           mality  mality  mality                                         
Pensil hardness                                                           
           8H      more    more  3H     3H                                
                   than    than                                           
                   9H      9H                                             
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 Note (1)-(3): same as in Table 1.                                        
 
    
     COMPARATIVE EXAMPLES 2 AND 3 
     For the comparison, paints were prepared quite in the same conditions as in Example 4 except for using acrylic resins of different glass transition temperature and hydroxyl values as shown in Table 2 to prepare coating materials (VIII) and (IX). Coated plates were prepared under the same conditions as those in Example 4 using these coating materials and the performance evaluation was practiced or carried out. The results are shown in Table 2. 
     EXAMPLE 7 
     Spherical α-alumina particles of 10 μm average particle size were added in 65% by weight converted as solid component into an acrylic solution prepared by polymerizing 99 parts of methyl methacrylate, one part of acrylic acid and 150 parts of toluene (acrylic resin having a glass transition temperature of 105° C. and a weight average molecular weight of 70,000) and stirred at a high speed to obtain a coating material (X). 
     The thus obtained coating material (X) was diluted with a thinner (20% by weight of toluene, 40% by weight of butanol and 40% by weight of methyl ethyl ketone), which was spray coated on an ABS resin plate to a thickness of 30 μm and then dried at 50° C. for 50 min. The results for the performance evaluation of the coating film on the plate obtained in this way are shown in Table 3. From the results, it can be seen that the coating material according to this invention shows extremely preferred abrasion resistance and scratched resistance. 
     COMPARATIVE EXAMPLE 4 
     A coating material (XI) was obtained by the same procedures as in the example 7 using a 40% by weight of toluene solution of n-butylmethacrylate polymer having a weight average molecular weight of 75,000 and a glass transition temperature of 20° C., which was coated on an ABS resin plate. The result for the performance evaluation of the coated film on the resin plate are shown in Table 3. 
     COMPARATIVE EXAMPLE 5 
     A coating material (XII) was obtained in the same procedures as in Example 7 except for using an acrylic resin having a weight average molecular weight reduced to 25,000, which was coated on an ABS resin plate. The results for the performance evaluation for the coated film on the resin plate are shown in Table 3. 
     
                       TABLE 3                                                     
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               Example                                                    
                      Comparative Example                                 
                 7        4         5                                     
Coating material (X)      (XI)      (XII)                                 
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Acrylic resin, glass                                                      
                 105       20       105                                   
transition temperature (°C.)                                       
Acrylic resin, average                                                    
                 70,000   75,000    25,000                                
molecular weight                                                          
Pencil hardness  7H       H         5H                                    
Abrasion resistance (mg) (1)                                              
                  85      185       169                                   
Adhesion property (2)                                                     
                 100      100        90                                   
Scratch resistance (3)                                                    
                 no abnor-                                                
                          entirely  entirely                              
                 mality   injured   injured                               
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 Notes (1)-(3): same as in Table 1.                                       
 
    
     EXAMPLE 8 AND COMPARATIVE EXAMPLES 6 AND 7 
     Spherical α-alumina particles of 10 μm average particle size were added to the acrylic resin solution obtained in Example 7 by 50% by weight (Example 8), 33% by weight (Comparative Example 6) and 87% by weight (Comparative Example 7) respectively converted as the solid component and stirred at a high speed to obtain coating materials (XIII), (XIV) and (XV). Each of the coating materials (XIII)-(XV) thus obtained was coated on an ABS resin plate and dried in the same procedures as in Example 7. The results for the performance evaluation for the coated films on the plates are shown in Table 4. The results for the coating material of Example 7 are also shown in Table 4. 
     
                       TABLE 4                                                     
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                        Comparative                                       
            Example     Example                                           
              7       8         6     7                                   
Coating material                                                          
              (I)     (XIII)    (XIV) (XV)                                
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α-alumina content (%)                                               
              65      50         33   87                                  
Pencil hardness (1)                                                       
              7H      6H        2H    7H                                  
Abrasion resistance                                                       
              85      97        190   160                                 
(mg) (2)                                                                  
Adhesion property (3)                                                     
              100     100       100   75                                  
Scratch resistance                                                        
              no      no        slightly                                  
                                      no                                  
              abnor-  abnor-    injured                                   
                                      abnor-                              
              mality  mality          mality                              
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 Notes (1)-(3): same as in Table 1.