Composite polyvinylchloride plastisol coating and method of applying it to a metal sheet
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of applying a composite polyvinylchloride plastisol coating to a metal sheet to produce a coating having high hardness and excellent impression resistance, bleed resistance and formability. The invention also relates to polyvinylchloride plastisols used in such method. 
     2. Description of the Prior Art 
     Polyvinylchloride plastisol coatings for metal sheets are widely applied for various uses because of the coatings&#39; superior formability, corrosion resistance, weatherability and capability of producing surface designs by embossing. However, conventional polyvinylchloride plastisol coatings for metal sheets have an inferior impression resistance and bleed resistance because of the softness of the coating. Therefore, the conventional polyvinylchloride plastisol coatings are easily deformed when the coated metal sheets are piled up in stacks, causing impressions to be formed in the embossed patterns of the coatings as well as bleeding of the coatings. 
     To improve upon the above-mentioned drawbacks of conventional polyvinylchloride plastisol coatings, the polyvinylchloride powders, plasticizers and diluents incorporated in the polyvinylchloride plastisols have been examined to obtain a harder coating, however, the improvements have been limited. 
     In the prior art, there are known methods relating to improvements of the above-mentioned drawbacks of polyvinylchloride plastisol coatings, for example, Japanese Patent Application Laid-Open No. Sho. 54-50549 and Japanese Patent Application Laid-Open No. Sho. 63-185630. However, the present invention is different from these inventions with regard to the components of the plastisol. 
     Japanese Patent Application Laid-Open No. Sho. 54-50549 relates to a polyvinylchloride plastisol coating for metal sheets having high hardness and superior bleed resistance, stain resistance and solvent resistance. The reference discloses a composite plastisol consisting of a conventional polyvinylchloride powder, reactive plasticizer and conventional plasticizer which is coated on a metal sheet and heated, and, in certain instances, is irradiated by ultraviolet rays. However, the hardness, impression resistance and bleed resistance of the coating are not completely satisfactory. 
     Japanese Patent Application Laid-Open No. Sho. 63-185630 also relates to a polyvinylchloride plastisol coating for metal sheets. This reference discloses a composite plastisol consisting of a specific polyvinylchloride powder and an acrylic resin powder which is coated on a metal sheet and heated. The present invention is different from this invention with regard to the materials used in the plastisol and with regard to the hardness, impression resistance and bleed resistance of the coating. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention thus provides a composite polyvinylchloride coating for a metal sheet which coating has high hardness, excellent impression resistance and excellent bleed resistance. 
     More specifically, the present invention provides a metal sheet covered with a composite plastisol comprising a specific polyvinylchloride powder having particle sizes which give 2 peaks in a distribution curve, an acrylic resin powder and an acrylic monomer and/or oligomer. In certain instances, the coated metal sheet is irradiated by ultraviolet rays after heating and embossing the coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 average particle size of the conventional polyvinylchloride power for a plastisol coating. 
     FIG. 2 average particle size of the claimed polyvinylchloride powder for a plastisol coating. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described below in more detail. 
     We shall begin with the composite polyvinylchloride plastisol coating. 
     The composite plastisol coating composition comprises a polyvinylchloride resin powder, an acrylic resin powder, an acrylic monomer and/or oligomer, a plasticizer, a stabilizer, and a heat setting initiator and a photoinitiator for the acrylic monomer and/or oligomer. The coating composition may also include agents such as pigments, fillers, etc. 
     Polyvinylchloride resin, which is one of the main components of the present invention, may include that which is polymerized by micro suspension, emulsion or solution methods. The resin should have an average degree of polymerization of 500 to 2,500 (i.e. the number of repeating monomeric units), and may include homopolymers, copolymers and partially cross-linked polymers. The monomers used for copolymerization with vinylchloride monomers are, for example, ethylene, propylene, vinyl acetate, vinyl propionate, vinyl stearate, alkyl vinyl ether, vinylidene chloride, di-ethyl fumarate and methacrylate. More than 95 wt % of the polyvinylchloride powder should be composed of particles whose sizes are in the range of 0.5 to 40 μm. Of that, 20 to 60 wt % should be particles whose sizes are in the range of 0.5 to 5 μm and 35 to 75 wt % should be particles whose sizes are in the range of 5 to 40 μm. The distribution curve corresponding to particle size should have 2 peaks in the 1 to  3 μm range and in the 8 to 15 μm range, respectively. 
     Although the average particle size of the conventional polyvinylchloride powder for a plastisol coating is 1 to 3 μm as shown in FIG. 1, the present inventors have found it desirable for the polyvinylchloride powder to be composed of particle sizes which give 2 peaks in the distribution curve as shown in FIG. 2. The present inventors have discovered that more than 5 wt % of polyvinylchloride powder having a particle size of more than 40 μm impairs the appearance of the coating surface. Less than 20 wt % of the polyvinylchloride powder having a particle size of 0.5 to 5 μm, or more than 75 wt % of the polyvinylchloride powder having a particle size of 5 to 40 μm is found to cause inferior gelation of the plastisol coating. On the other hand, more than 60 wt % of the polyvinylchloride powder having a particle size of 0.5 to 5 μm or less than 35 wt % of the polyvinylchloride powder having a particle size of 5 to 40 μm is found to cause high viscosity of the plastisol and thus requires great quantities of plasticizer or diluent to coat metal sheets and to impart softness to the coating. 
     Acrylic resin powder, which is another one of the main components according to the present invention, may include that which is polymerized by the micro suspension or emulsion methods, and may include homopolymers of methylmethacrylate, copolymers and partially cross-linked copolymers. The monomers used for copolymerization with methylmethacrylic acid are, for example, vinyl monomers having such functional groups as epoxy, carboxyl, methylol, alkyl amino and amido groups, and multifunctional compounds such as divinyl benzene, ethyleneglycol dimethacrylate and trimethylolpropane trimethacrylate monomers. In the present invention, acrylic resin powder having an average degree of polymerization of 500 to 30,000 is desirable. Acrylic resin polymers having an average degree of polymerization of less than 500 impair the formability of the coating, and polymers having an average degree of polymerization of more than 30,000 impair the gelation of the plastisol coating and cause brittleness of the coating. In these powders, a particle size of more than 0.1 μm and less than 40 μm is desirable. An acrylic resin particle size of more than 40 μm results in inferior gelation of the coating, and a particle size of less than 0.1 μm causes an high viscosity of the plastisol. 
     Acrylic monomers or oligomers, which are another main component according to the present invention, may be liquid chemicals having at least one kind of functional group in a molecule such as acryloyl, methacryloyl and vinyl, and may be polymerized by heating and irradiating with ultraviolet rays. Acrylic monomers which may be used in the present invention are, for example, 2-ethylhexyl acrylate, tetrahydrofurfuryl acrylate, neopentylglycol diacrylate, tri-methylolpropane triacrylate and pentaerytritol triacrylate. Acrylic oligomers which may be used in the present invention are, for example, polyol acrylate, polyesteracrylate, epoxy acrylate and urethane acrylate. A mixture of the above-mentioned acrylic monomers and oligomers may also be used in the present invention. 
     The heat setting initiator for acrylic monomers and oligomers may include several kinds of organic peroxides, and the photoinitiator for acrylic monomers and oligomers may include, for example, benzoin benzyl, benzoin ethylether and benzophenone. A mixture of the above-mentioned heat setting initiators and photoinitiators may also be used, and the amount should be 0.1 to 5 parts per 100 parts of the acrylic monomers and/or oligomers. 
     Plasticizers, stabilizers, pigments and other additives, which are additional components of the present invention, may include those used in conventional polyvinylchloride plastisol coatings. 
     The composition of the composite plastisol is as follows: 
     
         ______________________________________                                    
polyvinylchloride powder 100 parts                                        
acrylic resin powder       5 ˜60                                    
acrylic monomer and/or oligomer                                           
                           5 ˜60                                    
heat setting initiator   0.05˜3                                     
photoinitiator           0.05˜3                                     
plasticizer              as needed                                        
stabilizer               as needed                                        
pigment                  as needed                                        
diluent                  as needed                                        
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     The above amounts and those amounts described in the specification refer to parts by weight, unless otherwise specified. 
     The amount of acrylic resin powder and acrylic monomer and/or oligomer must not be over 60 parts or under 5 parts of either component, because the addition of over 60 parts impairs the embossing finish, while the addition of less than 5 parts has no effect. 
     Next, the process for obtaining the coated metal sheets of the present invention will be described. 
     A steel sheet, an electrogalvanized steel sheet, a hot dip galvanized steel sheet or an electrolytic chromate treatment steel sheet, etc., are used as the substrate steel sheet. A well known adhesive for polyvinylchloride-metal coated steel, such as an acrylic-type, epoxy-type, urethane-type or a rubber-type, etc., is used as the adhesive. The substrate steel sheet is typically precoated with adhesive, baked by a heat treatment and covered with the composite polyvinylchloride plastisol of the present invention. The substrate steel sheet with adhesive is coated with this composite polyvinylchloride plastisol by roll coating, dipping, knife coating or curtain coating, etc., and then baked at 150° to 250° C. for gelation and adhesion, after which the embossing finish is applied. In certain instances, the coated metal sheet is irradiated by ultraviolet rays after the heating and embossing the coating. The wavelength of the ultraviolet rays may be 200 to 400 μm and the ultraviolet rays may be generated by a mercury lamp, a xenon electronic-flash lamp or a carbon arc lamp. The irradiation should be done after heating and embossing the coating, because irradiation done before heating and embossing the coating causes an inferior embossing surface. 
     The detailed effects of the present invention will be further explained by the following examples. 
     EXAMPLES 
     Example 1 was produced by the following procedure: 
     A Zn-Co-Mo coelectrogalvanized steel sheet of 0.5mm thickness was coated with an epoxy-type adhesive 5 μm in thickness by a bar coater and heated in a hot air oven for 30 seconds at 300° C. The temperature of the steel sheet was 230° C. Then the steel sheet was cooled. The steel sheet was then coated with the composite polyvinylchloride plastisol shown in Table 1 150 μm in thickness by a knife coater and heated in a hot air oven for 60 seconds at 260° C. and immediately thereupon received an embossing finish by an engraved pebbling roll having a roughness of 7.1 μm in average roughness and 45.0 μm in maximum roughness. 
     Examples 2-6 of the present invention were produced according to the same procedure as Example 1, except for the substitution of the composite polyvinylchloride plastisol compositions shown in Table 1 and the irradiation of the coating surface by ultraviolet rays (Lamp: H2000L/6,Toshiba Denzai Co., Ltd.) after heating and embossing the coating. 
     The evaluation of the properties of the coatings described in the Examples are summarized in Table 3. 
     COMPARATIVE EXAMPLES 
     Comparative Example 1 was produced according to the same procedure as Example 1, except for the substitution of the composite polyvinylchloride plastisol composition shown in Table 2. 
     Comparative Example 2 was performed according to Japanese Laid-Open Patent Application No. Sho. 54-50549 and was produced by the same procedure as Example 1, except for the substitution of the composite polyvinylchloride plastisol composition shown in Table 2. 
     Comparative Examples 3 and 5 were also produced by the same procedure as Example 1, except for the substitution of the composite polyvinylchloride plastisol compositions shown in Table 2. 
     Comparative Example 4 was performed according to Japanese Laid-Open Patent Application No. Sho. 63-185630 and was produced by the same procedure as Example 2, except for the substitution of the compositive polyvinylchloride plastisol composition shown in Table 2. 
     The evaluation of the properties of the Comparative Examples are summarized in Table 3. 
     As mentioned in Table 3, the composite polyvinylchloride plastisol coated steel sheet according to the present invention has a coating of high hardness and shows excellent formability, embossing finish, impression resistance and bleed resistance. 
     The polyvinylchloride powders and acrylic resin powders used in the Examples and Comparative Examples are described in detail as followed: 
     PVC A : polyvinylchloride powder 
     average degree of polymerization(P); 1,650 
     distribution in particle size; 
     0.5 to 5 μm: 30 wt % 
     5 to 40 μm: 70 wt % 
     2 peaks distribution curve; 2 μm and 10 μm 
     PVC B: polyvinylchloride-vinylacetate powder; vinylacetate 5 wt % 
     average degree of polymerization (P); 1,300 
     distribution in particle size; 
     0.5 to 5 μm: 45 wt % 
     5 to 40 μm: 55 wt % 
     2 peaks distribution curve; 2 μm and 10 μm 
     Acrylic resin A: polymethylmethacrylate powder 
     average degree of polymerization(P); 20,000 
     distribution in particle size; 
     0.5 to 5 μm: 95 wt % 
     Acrylic resin B: polymethylmethacrylate powder 
     average degree of polymerization(P); 7,000 
     distribution in particle size; 
     3 to 20 μm: 95 wt % 
     Acrylic resin C: cross-linked polymethylmethacrylate powder 
     distribution in particle size; 
     3 to 30 μm: 95 wt % 
     The polyvinylchloride coated steel sheets, which were prepared in Examples 1-6 and Comparative Examples 1-5, were evaluated by the following test methods. The results are shown in Table 3. 
     (1) Viscosity of the polyvinylchloride plastisol was measured by a BM-type viscotester (Tokyo Keiki Co., Ltd.). 
     (2) Hardness of the coating was measured by a A-type duro meter (Shimadzu Co., Ltd). 
     (3) Formability of the polyvinylchloride coated steel sheet. 
     A sample steel sheet was subjected to the Dupont impact test (1/2&#34;×1 kg×50 cm) at temperatures of -5° C., 0° C. and 5° C. The sheets were evaluated and the following symbols are used to describe the results of the test. 
     O: no cracking in the coating at -5° C. 
     Δ: cracking at -5° C. and no cracking at 0° C. and 5° C. 
     X: cracking at -5° C., 0° C. and 5° C. 
     (4) Glossiness of the coating 
     60° specular glossiness was measured by a glossimeter UGX-5 (Suga Test Instruments Co., Ltd.) 
     (5) Embossing of the coating 
     A sample steel sheet was subjected to the surface roughness tester Surfcom 60A (Tokyo Seimitsu Co., Ltd.), and average roughness (Ra) and maximum roughness (Rmax) were shown. 
     (6) Impression resistance 
     Sample steel sheets, 15×15 cm in size, were piled up and were subjected to the load test (40° C.×3 kg/cm 2  ×10 day). After that, the embossing pattern of the coating was evaluated. 
     O : nothing unusual on the embossing pattern 
     Δ: embossing pattern is partially deformed 
     X: embossing pattern is deformed heavily 
     (7) Bleed resistance 
     Sample steel sheets and polyester films of 20μm in thickness, 15×15 cm in size, were piled up one after another and were subjected to the load test (50° C.×10kg/cm 2  ×10 day). After that, bleeded materials on the polyester film surface from the coating were observed. 
     O: no bleeding 
     Δ: slight bleeding 
     X: heavy bleeding 
     
                       TABLE 1                                                     
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                                    Plastisol                             
                                    viscosity                             
Example                                                                   
       Plastisol composition                                              
                            (Parts) (Poise)                               
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1      PVC A                87      260                                   
       Acrylic resin A      13                                            
       Acrylic oligomer(Aronix M-8060,                                    
                            15                                            
       Toagosei Co., Ltd.)                                                
       Organic peroxide(Perhexsa                                          
                            0.3                                           
       3M,Nipponushi Co., Ltd.)                                           
       Plasticizer (DOP)    30                                            
       Plasticizer (CS-16,Chisso Co.,Ltd.)                                
                            10                                            
       Stabilizer(Pb-type)  2                                             
       Pigment (Carbon black)                                             
                            1.5                                           
2      PVC A                91      250                                   
       Acrylic resin A      9                                             
       Acrylic oligomer(Aronix M-8060,                                    
                            15                                            
       Toagosei Co., Ltd.)                                                
       Photoinitiator (Darocure No.1116,                                  
                            0.45                                          
       Merk Co., Ltd.)                                                    
       Plasticizer (DOP)    30                                            
       Plasticizer (CS-16,Chisso Co.,Ltd.)                                
                            10                                            
       Stabilizer(Pb-type)  2                                             
       Pigment (Carbon black)                                             
                            1.5                                           
3      PVC A                80      260                                   
       Acrylic resin A      20                                            
       Acrylic monomer (TMPT,                                             
                            10                                            
       Shinnakamura Chemical Co., Ltd.)                                   
       Photoinitiator (Darocure No.1116,                                  
                            0.35                                          
       Merk Co., Ltd.)                                                    
       Organic peroxide(Perhexsa                                          
                            0.05                                          
       3M,Nipponushi Co., Ltd.)                                           
       Plasticizer (DOP)    30                                            
       Plasticizer (CS-16,Chisso Co.,Ltd.)                                
                            10                                            
       Stabilizer(Pb-type)  2                                             
       Pigment (Carbon black)                                             
                            1.5                                           
4      PVC A                77      250                                   
       Acrylic resin B      23                                            
       Acrylic oligomer(Aronix M-400,                                     
                            15                                            
       Toagosei Co., Ltd.)                                                
       Photoinitiator (Darocure No.1116,                                  
                            0.45                                          
       Merk Co., Ltd.)                                                    
       Plasticizer (DOP)    30                                            
       Plasticizer (CS-16,Chisso Co.,Ltd.)                                
                            10                                            
       Stabilizer(Pb-type)  2                                             
       Pigment (Carbon black)                                             
                            1.5                                           
5      PVC A                77      200                                   
       Acrylic resin C      23                                            
       Acrylic oligomer(Aronix M-400,                                     
                            15                                            
       Toagosei Co., Ltd.)                                                
       Photoinitiator (Darocure No.1116,                                  
                            0.45                                          
       Merk Co., Ltd.)                                                    
       Plasticizer (DOP)    30                                            
       Plasticizer (CS-16,Chisso Co.,Ltd.)                                
                            10                                            
       Stabilizer(Pb-type)  2                                             
       Pigment (Carbon black)                                             
                            1.5                                           
6      PVC B                77      300                                   
       Acrylic resin A      23                                            
       Acrylic oligomer(Aronix M-8060,                                    
                            15                                            
       Toagosei Co., Ltd.)                                                
       Photoinitiator (Darocure No.1116,                                  
                            0.45                                          
       Merk Co., Ltd.)                                                    
       Plasticizer (DOP)    45                                            
       Stabilizer(Organic Sn compound)                                    
                            2                                             
       Pigment (Titanium white)                                           
                            5                                             
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                       TABLE 2                                                     
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                                    Plastisol                             
Comparative                         viscosity                             
Example  Plastisol composition                                            
                            (Parts) (Poise)                               
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1        PVC A              100     300                                   
         Plasticizer (DOP)  30                                            
         Plasticizer (CS-16,Chisso Co.,                                   
                            15                                            
         Ltd.)                                                            
         Stabilizer(Pb-type)                                              
                            2                                             
         Pigment (Carbon black)                                           
                            1.5                                           
2        PVC A              100     280                                   
         Acrylic oligomer(Aronix                                          
                            15                                            
         M-8060,                                                          
         Toagosei Co., Ltd.)                                              
         Photoinitiator (Darocure                                         
                            0.60                                          
         No.1116, Merk Co.,Ltd.)                                          
         Plasticizer (DOP)  35                                            
         Stabilizer(Pb-type)                                              
                            2                                             
         Pigment (Carbon black)                                           
                            1.5                                           
3        PVC A              100     260                                   
         Acrylic monomer(TMPT,                                            
                            20                                            
         Shinnakamura Chemical                                            
         Co.,Ltd.)                                                        
         Organic peroxide(Perhexsa 3M,                                    
                            0.6                                           
         Nipponushi Co.,Ltd.)                                             
         Plasticizer (DOP)  30                                            
         Stabilizer(Pb-type)                                              
                            2                                             
         Pigment (Carbon black)                                           
                            1.5                                           
4        PVC A              80      290                                   
         Acrylic resin A    20                                            
         Plasticizer (DOP)  30                                            
         Plasticizer (CS-16,Chisso Co.,                                   
                            10                                            
         Ltd.)                                                            
         Stabilizer(Pb-type)                                              
                            2                                             
         Pigment (Carbon black)                                           
                            1.5                                           
5        PVC B              100     330                                   
         Plasticizer (DOP)  45                                            
         Stabilizer(Organic Sn                                            
                            2                                             
         compound)                                                        
         Pigment (Titanium white)                                         
                            5                                             
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                                           TABLE 3                                 
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      Film                                                                
Sample                                                                    
      Hardness                                                            
           Form-   Embossing  Impression                                  
                                    Bleed                                 
No.   (Duro A)                                                            
           ability                                                        
               Gloss                                                      
                   Ra(mm)                                                 
                        Rmax(mm)                                          
                              resistance                                  
                                    resistance                            
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Example                                                                   
1     94   O   6   3.3  29.6  O     O                                     
2     95   O   6   3.5  30.5  O     O                                     
3     97   Δ                                                        
               5   3.3  29.5  O     O                                     
4     95   O   7   3.6  31.5  O     O                                     
5     92   O   4   3.8  35.6  O     O                                     
6     87   O   10  3.6  32.5  O     O                                     
Compara-                                                                  
tive                                                                      
Example                                                                   
1     90   Δ                                                        
               10  3.5  30.3  X     X                                     
2     92   X   12  3.4  30.0  X     Δ                               
3     91   X   11  1.7  17.5  X     Δ                               
4     92   O   6   2.6  26.5  Δ                                     
                                    Δ                               
5     76   O   13  3.7  32.7  X     X                                     
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