Anticorrosive pigments on the basis of tertiary alkaline-earth aluminum phosphates and a process for the production thereof
Anticorrosive pigments containing phosphate, particularly zinc phosphate, are nowadays used as pigments for anticorrosive coatings, since they have good corrosion-inhibiting properties. 
     The law on environmental protection restricts the use of pigments containing heavy metals insofar as heavy metal contents may not exceed certain limits in the waste to be disposed and waste water produced by the production plants of paint manufacturers or in the water resulting from processing (spray booths). Corresponding technical steps involving costs have to be taken in order to prevent zinc from getting into the waste water, for example. Although the aluminum-zinc phosphate hydrates and/or the basic aluminum-zinc phosphate hydrates according to European patent 0 054 267, show a reduced solubility of zinc, it is still difficult to observe the stipulated limits. 
     In addition to the phosphates on the basis of heavy metals, patent literature describes secondary phosphates of the alkaline-earth metals as anticorrosive pigments. However, their efficiency cannot be compared with zinc phosphate. For example, calcium or magnesium hydrogenphosphates are improved, according to European patent 0 028 290, by admixing zinc oxide, while the anticorrosive pigments which are known from DE-PS 37 31 737 and based upon alkaline-earth hydrogenphosphates having a content of alkaline-earth carbonates, primarily protect against filiform corrosion on aluminum. 
     The object of the present invention is to develop heavy metal-free phosphate pigments on the basis of alkaline-earth metals, which can be used for anticorrosive coatings in general, i.e. also for steel, and are more efficient than the known zinc phosphates. When examining the corrosion protection of phosphate piqments contained in a primer coating common in practice, it was found that, in combination with aluminum, phosphates on the basis of magnesium and/or calcium have better anticorrosive properties than secondary phosphates, on the basis of calcium or magnesium, and tertiary phosphates, on the basis of magnesium, calcium and zinc. 
     Accordingly, the problem is solved by anticorrosive pigments on the basis of tertiary alkaline-earth aluminum phosphates, with only magnesium and calcium, of course, coming into consideration as alkaline earths for practical reasons. 
     These anticorrosive pigments are produced in the known per see manner from alkaline-earth and aluminum compounds sufficiently soluble in phosphoric acid as well as phosphoric acid, e.g. alkaline-earth hydrogen or carbonate, aluminum hydroxide and phosphoric acid, at a preferably slightly elevated temperature, a mixed crystal forming from aluminum phosphate and alkaline-earth phosphate. As regards the alkaline-earth phosphate, Ca 3  (PO 4 ) 2 .n H 2  O and/or Ca 5  (PO 4 ) 3  OH (hydroxylapative) could be proved for calcium by means of X-ray affraction. When the aluminum and/or alkaline-earth contents are increased, aluminum oxide hydrate and/or alkaline-earth hydroxide may also be present. The resulting pigments are filtered off, dried as usual and preferably also ground depending on the intended use. Drying is carried out preferably at 100°-110° C., e.g. at about 105° C. If desired, calcining may also be carried out, a temperature of at least about 150° C. usually being sufficient. However, a calcination temperature of about 600° C. or above may also be applied. 
     As mentioned earlier, only magnesium and calcium come into consideration for practical reasons. For the sake of simplicity, the expression alkaline earths (AE) is used hereinafter, even though only these two alkaline earths are of interest. 
     The pigments may be represented by the formula AE x  Al y  (PO 4 ) z . Hereinafter, the ratio x:y:z is referred to as the atomic ratio or numerical atomic ratio which may also be expressed by the formula AE x  Al y  P z . It is repeated expressly that this does not include the atomic weights but, as common in the chemical spelling of formulae, only the number of atoms or atom groups (such as PO 4 , which, of course, can also be represented only by P as well). The composition may then be given in percent by the percentage of the individual components with respect to the total number of all components. The mixed crystal Ca 3  (PO 4 ) 2 .AlPO 4  =Ca 3  Al(PO 4 ) 3  is mentioned by way of example. It has an atomic ratio or atom group ratio of 3 Ca to 1 Al to 3 P or PO 4 . In this case, the percentage of the individual components is 43% of Ca, 14% of Al and 43% of P or PO 4  . 
     In the inventive anticorrosive on the basis of tertiary alkaline-earth aluminum phosphates, the ratio of the three components alkaline earth to aluminum to phosphorus, expressed in the above-defined atomic ratio of x:y:z, is generally 40-70% % of alkaline earth to 5-30% of aluminum to 20-50 % of phosphorus or phosphate, the amounts, of course, being chosen such that 100% result each time. This ratio is preferably 40-65% of alkaline earth to 7-25% of aluminum to 25-45% of phosphorus, and a ratio of 50-65% of alkaline earth to 8-15% of aluminum to 25-35% of phosphorus is particularly preferred. 
     Thus, it is apparent that relatively more alkaline earth than aluminum and phosphorus is present in the particularly preferred range. The anticorrosive effect is improved when the amount of magnesium and calcium is relatively higher than that of aluminum and phosphorus, but, also within the general ratio, it is markedly above the anticorrosive effect of e.g. zinc phosphate. 
    
    
     The following examples explain the invention and show the results of the anticorrosive test. 
     EXAMPLE 1 
     A magnesium-aluminum phosphate hydrate forms when reacting magnesium oxide and/or magnesium carbonate as well as aluminum hydroxide in 10-40% aqueous suspension with conventional industrial phosphoric acid (about 75% by weight). 
     1 mole of magnesium oxide and 0.2 mole of aluminum hydroxide are used together with 0.6 mole of commercial industrial phosphoric acid (about 75% by weight). The reaction results in slight heating so that, starting from room temperature, a reaction temperature from 30° to 60° C. adjusts depending on the amount of batch. A magnesium aluminum phosphate hydrate forms, whose atomic ratio of magnesium to aluminum to phosphorus is 1.66:0.33:1, i.e. 56% 11% 33% when expressed in percent. The pigment obtained in the reaction batch is filtered off as usual, dried and, depending on intended use, optionally ground. Each of the batches according to the examples was dried at 105° C. Calcining may optionally be carried out as well. 
     EXAMPLE 2 
     A calcium-aluminum phosphate hydrate forms when reacting calcium hydroxide and aluminum hydroxide in 10-40% aqueous suspension with conventional industrial concentrated phosphoric acid (75% by weight). 
     1 mole of calcium hydroxide and 0.16 mole of aluminum hydroxide are reacted with 0.5 mole of phosphoric acid (75 %) at temperatures from 30° to 60° C. Aluminum phosphate forms a mixed crystal together with calcium phosphate which, when investigated by means of X-ray diffraction, consists of Ca 3  (PO 4 ) 2 .n H 2  O, data card No. 18-303, edition 1986, or Ca 5  (PO 4 ) 3  OH (hydroxylapatite, data card No. 0-436, edition 1986). The atomic ratio of calcium to aluminum to phosphorus is 2:0.3:1. The precipitated pigment is further treated as indicated in Example 1. 
     A mixture of alkaline earths may also be present in the alkaline-earth aluminum phosphates using a mixture of the alkaline-earth compounds. 
     Correspondingly, instead of adding the phosphoric acid to the suspension of the alkaline-earth hydroxide or carbonate and the aluminum hydroxide, the aluminum hydroxide, for example, may be dissolVed in the phosphoric acid, and then this component may be added to the suspension of the alkaline-earth compound or, vice versa, the suspension of the alkaline-earth compound may be added to the reaction mixture consisting of aluminum hydroxide and phosphoric acid. For this purpose, the phosphoric acid used is usually the common industrial type having about 75% by weight, since it is the most inexpensive and meets the requirements for purity stipulated herein. 
     In addition to the pigments used in Examples 1 and 2, which are referred to as pigments 2 and 5 in Table 1 below, the other pigments of this Table were also obtained as according to Examples 1 and 2. 
     The investigation and evaluation of the anticorrosive behavior of these pigments were carried out as follows: 
     The anticorrosive pigments of the following Table 1 were investigated in a primer coating in the salt spray test and humidity test with respect to their anticorrosive effect in comparison with tertiary magnesium, calcium and zinc phosphate as well as a secondary calcium phosphate. 
     
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Binding agent    short-oil alkyd resin                                    
PVC/CPVC (pigment volume                                                  
concentration/critical                                                    
pigment volume concentration                                              
                 0.7                                                      
formulation example                                                       
                 see Table 2                                              
dry film thickness                                                        
                 45 +/- 5 microns                                         
drying conditions                                                         
                 7 days at room temperature                               
                 and 2 hours at 50° C.                             
anticorrosive test                                                        
                 Salt spray test, ASTM D714-56                            
                 after 360 hours                                          
                 Humidity test, ASTM D2247-68                             
                 after 500 hours                                          
evaluation       30 minutes after weathering.                             
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     In order to evaluate the anticorrosive behavior, data conforming to the standards, such as degree of blistering according to ASTM D 714-56, creepage at the scribe according to ASTM D 1654-75 and corrosion of the metal surface according to ASTM D 610-68, is rated according to the enclosed evaluation pattern of Tables 3 and 4 and combined to an overall evaluation (rating) (100=best anticorrosive value, 0=poorest anticorrosive value). 
     In order to accurately determine the degree of rusting, the coatings were removed by pickling from the base material by means of methylene chloride after weathering. 
     
                                           TABLE 1                                 
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Anticorrosive test - results of short-term weathering                     
                    Overall rating                                        
                    Salt spray test ASTM D 714-56 after                   
                    360 hours             Humidity test ASTM D 2247-68    
                          Creepage at                                     
                                 Degree of                                
                                          after 500 hours                 
                    Degree of                                             
                          the scribe                                      
                                 rating            Degree of              
               atomic                                                     
                    blistering                                            
                          ASTM   ASTM     Degree of                       
                                                   rusting                
               ratio                                                      
                    ASTM  D 1654-74                                       
                                 D 610-68                                 
                                       rat-                               
                                          blistering                      
                                                   ASTM D                 
                                                            rat-68        
               x:y:z                                                      
                    D 714-56                                              
                          (in mm)                                         
                                 (in %)                                   
                                       ings                               
                                          ASTM D 714-56                   
                                                   (in %)   ings          
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    Zn.sub.3 (PO.sub.4).sub.2.x H.sub.2 O                                 
                    80                                                    
                      (8F)                                                
                          65 (2) 20 (33)                                  
                                       46 50  (6M) 0   (50) 17            
    Mg.sub.3 (PO.sub.4).sub.2.x H.sub.2 O                                 
                    80                                                    
                      (8F)                                                
                          70 (1,5)                                        
                                 20 (33)                                  
                                       48 65  (4F) 0   (50) 22            
    CaHPO.sub.4 .x H.sub.2 O                                              
                    65                                                    
                      (4F)                                                
                          35 (8) 20 (33)                                  
                                       35 45  (0)  0   (50) 15            
    Ca.sub.5 (PO.sub.4).sub.3 OH                                          
                    65                                                    
                      (4F)                                                
                          80 (1) 20 (33)                                  
                                       46 45  (4M) 0   (50) 15            
Pig. 1                                                                    
    Mg.sub.x Al.sub.y (PO.sub.4).sub.z.n H.sub.2 O                        
               1:0,3:1                                                    
                    70                                                    
                      (&lt;4F)                                               
                          80 (1) 20 (33)                                  
                                       48 55  (8M) 20  (33) 31            
Pig. 2                                                                    
      &#34;        1,66:0,3:1                                                 
                    70                                                    
                      (&lt;4F)                                               
                          70 (1.5)                                        
                                 60 (10)                                  
                                       65 55  (8M) 40  (16) 45            
Pig. 3                                                                    
    Ca.sub.x Al.sub.y (PO.sub.4).sub.z.n H.sub.2 O                        
               1:0,3:1                                                    
                    60                                                    
                      (2-4F)                                              
                          65 (2) 20 (33)                                  
                                       41 70  (6F) 0   (50) 23            
Pig. 4                                                                    
      &#34;        1,7:0,3:1                                                  
                    70                                                    
                      (6F)                                                
                          65 (2) 70 (3)                                   
                                       69 55  (8M) 60  (10) 58            
Pig. 5                                                                    
      &#34;        2:0,3:1                                                    
                    60                                                    
                      (2-4F)                                              
                          70 (1,5)                                        
                                 40 (16)                                  
                                       53 30  (6MD)                       
                                                   60  (10) 50            
Pig. 6                                                                    
      &#34;        2,5:0,3:1                                                  
                    65                                                    
                      (4F)                                                
                          35 (8) 20 (33)                                  
                                       35 65  (4F) 0   (50) 22            
Pig. 7                                                                    
      &#34;        2:0,4:1                                                    
                    60                                                    
                      (2-4F)                                              
                          80 (1) 70 (3)                                   
                                       70 35  (8MD)                       
                                                   60  (10) 52            
Pig. 8                                                                    
      &#34;        2:0,8:1                                                    
                    70                                                    
                      (&lt;4F)                                               
                          80 (1) 60 (10)                                  
                                       68 55  (8M) 40  (16) 45            
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     When investigating corrosion protection by means of the salt spray test, anticorrosive properties were obtained by the new anticorrosive pigments, which were better than those of secondary calcium phosphate and tertiary magnesium, calcium and zinc phosphate. Particularly, the degree of rusting was markedly reduced on the metal surface. 
     As shown in the above Table 1 containing the results of short-term weathering, the humidity test also revealed that the degree of rusting is reduced by the new pigments. 
     Below, Table 2 shows the formulation for the production of a primer coating by means of pigment 4 serving as an example; Tables 3 and 4 give the rating pattern for the results of Table 1. 
     
                       TABLE 2                                                     
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Formulation:        parts by weight %                                     
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short-oil alkyd resin, 60%                                                
                    46.8                                                  
(34% of special fatty acids)                                              
modified montmorillonite as an                                            
                    0.4                                                   
anti-settling agent                                                       
glycol ether        1.7                                                   
xylene              11.3   premix bin-                                    
                           ders, solvents                                 
                           and additives                                  
calcium octoate 10% Ca                                                    
                    0.1                                                   
pigment 4           5.4                                                   
natural barium sulphate                                                   
                    17.4   disperse in                                    
                           a pearl mill                                   
magnesium silicate  9.8                                                   
titanium dioxide rutile                                                   
                    5.6                                                   
lead-free siccative 1.2    stir in sep-                                   
                           arately one af-                                
                           ter the other                                  
antioxidants        0.3                                                   
                    100.0                                                 
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PVC in %                = 31                                              
PVC/CPVC                = 0.7                                             
non-volatile content in %                                                 
                        = 67                                              
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                       TABLE 3                                                     
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TABLE OF EVALUATION                                                       
                        Creepage at the                                   
          Degree of blister-                                              
                        scribe given in mm                                
Rating    ing ASTM D 714-56                                               
                        ASTM D 1654-74                                    
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100-90    --              0-0.4                                           
90-60     8 F-2 F       0.8-3.2                                           
55-40     8 M-2 M       4.8-6.4                                           
35-20     8 MD-2 MD      9.5-12.7                                         
15-0      8 D-2 D       15.9-25                                           
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                       TABLE 4                                                     
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                 degree of rusting                                        
rating           ASTM D 610-68                                            
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100    very good     no corrosion or less than 0.01%                      
80     good          spot-like corrosion, but less                        
                     than 1-3%                                            
60     good - moderate                                                    
                     10% of the surface corroded                          
40     moderate      16% of the surface corroded                          
20     poor          33% of the surface corroded                          
 0     very poor     50% of the surface corroded                          
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     Overall Rating 
     To obtain the overall rating according to salt spray and humidity test weathering, the individual ratings are added and divided by 4 and 3, respectivley, the ratings mentioned in TABLE 4 being counted twice because of their greater importance.