Anionic polymers containing 3-acrylamido-3-methylbutanoic acid as scale inhibitors for preventing or reducing calcium phosphate and other scales
INTRODUCTION 
     Certain boiler waters and many industrial waters such as those used in the operation of cooling towers are treated with a variety of inorganic and organic phosphorous-containing compounds. Such treatments tend to produce calcium phosphate scales which adhere to the metal surfaces of boilers and metallic heat exchangers. 
     Many of the known organic scale inhibitors and scale dispersants, both inorganic and those containing water-soluble polymers, while being effective against a wide variety of scales, are not entirely effective against calcium phosphate scales. 
     Pure calcium phosphate scales may exist as such but frequently are found as contaminants of calcium carbonate and calcium or magnesium salt scales. When such scales contain at least 10% of calcium phosphate, they are suitable for treatment with the scale inhibitors of the invention. 
     THE INVENTION 
     A method of inhibiting scale, particularly calcium phosphate scale and including calcium and magnesium carbonate scale and slightly soluble zinc compounds, which are formed on metal surfaces in contact with industrial cooling waters, evaporated sea water, oil well drilling waters, and boiler waters, which comprises treating the water present in such systems with a few ppm of a water-soluble anionic acrylic acid polymer which contains from 10-80 mole percent and preferably 20-50 mole percent of 3-acrylamido-3-methylbutanoic acid (AMBA). These polymers have a weight average molecular weight within the range of 5-100,000 with the preferred molecular weight being 10-20,000. 
     The anionic portion of the polymer is preferably acrylic acid, although the acrylic acid may be substituted in whole or in part by other anionic monomers such as methacrylic acid, maleic acid or its anhydride, fumaric acid and itaconic acid. Such terpolymers are included within the definition of &#34;acrylic acid polymers&#34;. 
     The polymers may be employed either in their free acid form or in the form of their water-soluble salts such as sodium, potassium, ammonia or amine. 
     The polymers are used to treat the waters which contain scale-forming amounts of calcium phosphate and related scale-forming materials by treating such waters with a few parts per million of the active polymer. By a &#34;few parts per million&#34; is meant at least 3 parts per million with a typical dosage range being between 5-50 parts per million. 
    
    
     A typical method for preparing polymers of the type described above is set forth below: 
     EXAMPLE 
     Copolymer of Acrylic Acid (80 Mole %) and 3-Acrylamido-3-Methylbutanoic Acid (20 Mole %) 
     An aqueous solution of acrylic acid (28.2 g, 0.392 mole) and 3-acrylamido-3-methylbutanoic acid (16.8 g, 0.098 mole) was prepared by stirring the monomers together with 225 g of deionized water until complete solution occurred. The monomer solution was transferred to a reaction vessel and the solution was stirred and purged with nitrogen while heating it to 60° C. over 15 minutes. A solution of ammonium persulfate (2.25 g dissolved in 5.25 g of deionized water) was added all at once to the monomer solution followed immediately by a solution of sodium metabisulfite (6.75 g dissolved in 15.75 g of deionized water). Initiation occurred rapidly as the temperature of the reaction rose to 80° C. in one minute. After approximately sixteen minutes, the exotherm was complete and the temperature of the reaction had returned to 60° C. This temperature was maintained at 60° C. for 3.5 hours longer while continuing nitrogen purging. At the end of this time the reaction mixture was cooled to room temperature. The resulting polymer solution contained less than 1 ppm residual acrylic acid. Molecular weight (weight average) of the polymer was 13,700 as determined by GPC using polystyrene sulfonate standards in aqueous solution. 
     Advantages of the Invention 
     AMBA polymers have calcium phosphate scale inhibition activity better than a commercial sulfonated styrene-maleic acid polymer (Versa TL-4) and better than a commercial acrylic acid-2-hydroxypropyl acrylate polymer both of which are commonly used in cooling water as Ca 3  (PO 4 ) 2  scale inhibitors (see Table I). 
     AMBA copolymers are hydrolytically stable in high pH applications and formulations because of the hindered secondary amide group in AMBA. Polymers containing ester functionality as in acrylic acid-2-hydroxypropyl acrylate copolymers, lose activity under high pH conditions as the ester groups hydrolyze. 
     AMBA copolymers have calcium carbonate scale inhibitor activity better than 1-hydroxy-ethylidenediphosphonic acid (HEDP) and better than low molecular weight commercial polyacrylic acids which are commonly used in industrial cooling water for CaCO 3  scale inhibition (see Table II). 
     Evaluation of the Invention 
     Following are laboratory test results showing several advantages of the invention. 
     
                       TABLE I                                                     
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Calcium Phosphate Inhibitor Tests.sup.1 of Polymers                       
Made with 3-Acrylamido-3-Methylbutanoic Acid (AMBA)                       
                                 %                                        
Sam-               Mole          Inhib.                                   
ple  Sample        Ratio   MW.sup.2                                       
                                 10 ppm                                   
No.  Composition   (%)     (GPC) Dosage                                   
                                       Comments                           
______________________________________                                    
1    Sulf. styrene 75:25   18,950                                         
                                 85    reference                          
maleic acid                      polymer                                  
2    AA/HPA        75:25    7,350                                         
                                 85    reference                          
                                       polymer                            
3    AA/AMBA       80:20   13,750                                         
                                 95                                       
4    AA/AMBA       50:50   11,500                                         
                                 89                                       
5    AA/MAA/AMBA   60:20:20                                               
                           15,100                                         
                                 79                                       
6    MAA/AMBA      80:20   27,500                                         
                                 37                                       
7    AM/AMBA       50:50    8,860                                         
                                  2                                       
8    Polyacrylic   100      4,000                                         
                                  0    reference                          
     acid                              polymer                            
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 .sup.1 All screening tests were done using water containing 250 ppm Ca (a
 CaCO.sub.3), 125 ppm Mg (as CaCO.sub.3), pH maintained at 8.5, no iron   
 added. Tests are run for 4 hrs then filtered through a 0.45 um filter to 
 remove Ca.sub.3 (PO.sub.4).sub.2 precipitate and the filtrate was analyze
 for remaining phosphate.                                                 
 .sup.2 Molecular weights were determined by GPC and are based on         
 polystyrene sulfonate standards in aqueous solution.                     
 Abbreviations                                                            
 AA -- acrylic acid                                                       
 MAA -- methacrylic acid                                                  
 AMBA -- 3acrylamido-3-methylbutanoic acid                                
 AM -- acrylamide                                                         
 HPA -- 2hydroxypropyl acrylate                                           
 
    
     
                       TABLE II                                                    
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Calcium Carbonate Scale Inhibitor Tests of Polymers                       
Made with 3-Acrylamido-3-Methylbutanoic Acid (AMBA)                       
                                 %                                        
Sam-               Mole          Inhib.                                   
ple  Sample        Ratio   MW    10 ppm                                   
No.  Composition   (%)     (GPC) Dosage                                   
                                       Comments                           
______________________________________                                    
8    Polyacrylic   100      4,000                                         
                                 42    reference                          
     acid                              polymer                            
9    HEDP          100       206 48    reference                          
                                       compound                           
10   AA/AMBA       80:20   13,700                                         
                                 60                                       
4    AA/AMBA       50:50   11,500                                         
                                  9                                       
6    MAA/AMBA      80:20   27,500                                         
                                 18                                       
5    AA/MAA/AMBA   60:20:20                                               
                           15,100                                         
                                 19                                       
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