Patent Publication Number: US-6217738-B1

Title: Tin plating electrolyte compositions

Description:
This invention relates to electrolyte compositions suitable for electroplating surfaces with tin, e.g., in high speed strip or wire plating processes. 
     Ideally an electrolyte should possesses the following properties: produce a wide plating range, give a good quality deposit, have low corrosivity, have good conductivity, have good antioxidant properties, have low toxicity and have low environmental impact. 
     Many electrolyte compositions have been made known and are available in the art. Typical baths include aqueous acidic baths based upon fluoroborate and fluorosilicate electrolytes as described, for example, in U.S. Pat. No. 3,769,182 and U.S. Pat. No. 4,118,289. Aryl sulphonic acids have been used in electroplating baths as disclosed, for example, in U.S. Pat. No. 3,905,878. Traditionally, the aryl sulphonic acid of choice is phenolsulphonic acid as used in the Ferrostan process. 
     Mineral acid (especially sulphuric acid) electroplating baths have also been described. For example, U.S. Pat. No. 2,156,427 describes a bath containing tar acids and sulphuric acid for producing tin coatings of a fine crystalline texture. 
     Alkane sulphonic acids containing 1 to 5 carbon atoms in the alkyl group have previously been used in certain electrolytic plating baths and were first disclosed for this use in U.S. Pat. No. 2,522,942. More recently, methanesulphonic acid has been claimed as a specific preferred example of an alkane sulphonic acid in combination with a number of brightening agents for use in the electroplating of tin, lead and tin-lead alloys for example as in U.S. Pat. No. 4,565,610 and U.S. Pat. No. 4,617,097. Systems based on methanesulphonic acid however suffer from high cost, toxicity, odour problems and have been known to introduce surface defects on flow brightened strip steel plate. 
     Various plating bath compositions comprising an alkane or alkanol sulphonic acid (normally methane sulphonic acid), a tin and/or a lead salt and various auxiliary additives are known. Known auxiliary additives range from smaller organic molecules to large polymeric surfactant molecules and are described in U.S. Pat. No. 4,555,314, U.S. Pat. No. 4,565,609, U.S. Pat. No. 4,582,576, U.S. Pat. No. 4,599,149, U.S. Pat. No. 4,617,097, U.S. Pat. No. 4,666,999, U.S. Pat. No. 4,673,470, U.S. Pat. No. 4,701,244, U.S. Pat. No. 4,828,657 and U.S. Pat. No. 4,849,059. 
     Plating bath compositions containing mixtures of aryl and alkyl sulphonic acids are also known in the art, for example, as described in EP 0 490 575. 
     Various addition agents have been proposed which enhance the quality of the tin plate. They can include condensates of hydrophobic organic compounds with alkylene oxides such as, for example, alpha naphthol 6 mol ethoxylate (‘ENSA 6’ as supplied by Emery-Trylon); alkylbenzene alkoxylates such as the ‘Tritons’; derivatives of N-heterocycles such as, for example, 2-alkylimidazolines; aromatic aldehydes such as naphthaldehyde; derivatives of 2,2-bis(4-hydroxyphenyl)propane, for example, as ‘Diphone V’ (supplied by Yorkshire Chemicals) formed by reacting 2,2-bis(4-hydroxyphenyl) propane with a sulphonating agent; and 2,4,6-substituted phenols in which at least one of the substituents includes a secondary, tertiary or quaternary nitrogen atom as described in U.S. Pat. No. 3,954,573. The latter addition agents have, however only been described for use in fluoroborate tin plating systems. 
     In the case of strip tinplate manufacture it is desirable from a commercial point of view to have a system capable of giving satisfactory tin deposits over as wide as possible a range of current densities to accommodate all variations in speed of production and minimise the incidence of current density defects. 
     Accordingly this invention provides a composition suitable for use in a process for electroplating surfaces with tin, which exhibits all the aforementioned benefits of wide plating range, good quality deposits and enhanced environmental benefits. 
     It comprises: 
     a) One or more acids selected from sulphuric acid, sulphamic acid, aryl sulphonic acids, alkyl sulphonic acids and alkanol sulphonic acids. 
     b) One or more addition agents comprising a mono-, di- or tri-substituted phenol (each optionally alkyoxylated) or a mixture of two or more such compounds, in which at least one of the substituents includes a secondary, tertiary or quaternary nitrogen atom. The phenol is preferably a 2,4 or 2,6-disubstituted or 2,4,6-trisubstituted phenol. 
     c) A tin source. 
     d) An antioxidant (optional). 
     e) Water. 
     Preferably tin source is a tin salt. Alternatively the tin source may be elemental tin. 
     The acid may be any single acid or combination of acids. Preferred examples of acids include sulphuric acid, sulphamic acid, phenolsulphonic acid, methane sulphonic acid and toluenesulphonic acid. 
     The acid or acids are present in a total concentration of 25-500 g/l of the composition (with respect to the composition), more preferably 30-250 g/l, even more preferably 30-100 g/l. 
     The tin source is preferably present in the composition at a concentration of 5-100 g/l (more preferably 15 to 60 g/l) with respect to the composition. Where a tin salt is the tin source it does not have to be a salt of the mono-substituted benzenesulphonic acid or inorganic acid. Thus the composition may contain ions other than tin, sulphonate and those from the inorganic acid. Where the tin source is solid tin, it may be as a tin anode which gradually dissolves as electrolysis proceeds to maintain a substantially constant concentration of tin ions in an electroplating bath. Where the tin source is a tin salt it may be metered to the electroplating bath so that as tin is electro-deposited from the bath, tin salt is added to the bath to maintain the concentration of tin ions in the bath at a constant level. 
     An antioxidant may optionally be added. These materials retard the oxidation of divalent tin to tetravalent tin which may lead to sludge formation and stannous tin loss. The preferred amount of antioxidant to be added is in the range 1 to 50 g/l of the composition and most preferably from 2.5 to 20 g/l of the composition. Typical antioxidants have been described for example in U.S. Pat. No. 3,749,649 and include 1,2,3-trihydroxybenzene, 1,2-dihydroxybenzene, 1,2-dihydroxybenzene-4-sulphonic acid, 1,2-dihydroxybenzene-3,5-disulphonic acid, 1,4-dihydroxybenzene, 1,4-dihydroxybenzene-2-sulphonic acid, 1,4-dihydroxybenzene-2,5-disulphonic acid or vanadium pentoxide. 
     The composition of the present invention also comprises one or more addition agents containing a mono-, di- or tri-substituted phenol (each optionally alkoxylated) in which at least one of the substituents includes a secondary, tertiary or quaternary nitrogen atom. The phenols are preferably 2,4 or 2,6-disubstituted or 2,4,6-trisubstituted phenols. 
     Addition agents have the general formula:                    
     in which: 
     Y=alkylene, CH 2 CH 2 O or CH(CH 3 )CH 2 O 
     n 3 =0-10 
     n 3 =1 when Y=alkylene 
     R=a group of formula                    
     in which: 
     R 3 =H, alkyl, aryl, hydroxyl or CHO, with the proviso that when R 3  is hydroxyl or CHO, n 2 =1-3, and when R 3  is H, alkyl or aryl, n 2  is 1, 
     R 4 =H, alkyl, cycloalkyl, hydroxyalkyl or alkoxyalkyl 
     R 5 =alkyl, optionally interrupted by O or N atoms, which may be further substituted, 
     X=a phenol (optionally alkoxylated) radical, optionally further substituted 
     n=0 or 1 
     n 1 =1 to 7 
     when n=0, n 1 =1 
     when n=1, n 1 =1-7 
     R may optionally be the quaternary ammonium salt formed by reaction with acids such as, sulphuric acid, toluenesulphonic acid, sulphamic acid, phenolsulphonic acid and methanesulphonic acid. 
     R 1  and R 2  (which may be the same or different)=R, H or optionally substituted alkyl. 
     Especially preferable examples include:                    
     As will be known to those skilled in the art such additives can conveniently be made by condensation of the phenol with an aldehyde and an amine under acidic or basic conditions. The molar proportions of phenol to amine to aldehyde may be varied over a wide range typically the range will be from 1:1:1 to 1:2:2. As will be obvious to those skilled in the art such a reaction will give rise to a mixture of monomeric and polymeric products. The reaction products may also be alkoxylated with either ethylene or propylene oxide. Although any phenol, amine and aldehyde may be used preferred examples include: 
     Phenols: ortho and/or para alkyl-phenols, where the alkyl group is methyl, ethyl, isopropyl, n-butyl, sec-butyl, tert-butyl, isoamyl, hexyl, and nonyl; Diphenols: 2,2-bis(4-hydroxyphenyl)propane or 4,4′-dihydroxydiphenylsulphone. 
     Aldehydes: formaldehyde, acetaldehyde, glyoxal. 
     Amines: methylamine, dimethylamine, ethylamine, diethylamine, n- or iso-propylamine, n- or sec-butylamine, n-hexylamine, ethanolamine, diethanolamine, n or iso-propanolamine, 2-aminobutanol, 4-aminobutanol, 2-amino-5-diethylaminopentane, 2-(2-aminoethoxy)ethanol, 2-(2-aminoethylamino)ethanol, 2-amino-2-ethyl-1,3-propandiol. 
     The compositions of the present invention when used for tin plating provides a wider plating range and brighter plates with less surface defects than can be obtained by using other addition agents. 
     The surfaces which can be tin-plated using the compositions and methods of the invention are those surfaces which may normally be tin-plated e.g. steel or copper. The plated surfaces include those of strips and wires and the applicable processes include those involving the plating of strips and wires. 
    
    
     EXAMPLES 
     Example 1 
     This example illustrates how the preferred addition agents may be synthesised using base catalysis. 
     Substituted phenol (1 mole), water (equivalent weight based on phenol) and sodium hydroxide (0.2 mole) were charged to the reactor. The mixture was heated at 60° C. with agitation until a clear solution was obtained. 
     Amine (2 moles) was charged into a second reactor and aldehyde (2 moles) slowly added with stirring whilst keeping the temperature below 60° C. This solution was added to the alkylphenol solution in this first reactor and the mixture heated at 100° C. for half an hour. 
     The following examples are illustrative of this method of synthesis. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Example 
                   
                   
                   
               
               
                 Number 
                 Phenol 
                 Amine 
                 Aldehyde 
               
               
                   
               
             
            
               
                 2 
                 4-Methylphenol 
                 Ethanolamine 
                 Formaldehyde 
               
               
                 3 
                 4-tert Butylphenol 
                 2-(2-aminoethylamino) 
                 Formaldehyde 
               
               
                   
                   
                 ethanol 
               
               
                 4 
                 4,4′-Isopropylidene- 
                 Diethanolamine 
                 Formaldehyde 
               
               
                   
                 diphenol 
               
               
                 5 
                 4-Ethylphenol 
                 iso-Propanolamine 
                 Formaldehyde 
               
               
                 6 
                 4-tert Butylphenol 
                 Diethanolamine 
                 Formaldehyde 
               
               
                 7 
                 4-tert Butylphenol 
                 Ethanolamine 
                 Formaldehyde 
               
               
                 8 
                 3-Methylphenol 
                 Ethanolamine 
                 Glyoxal 
               
               
                 9 
                 4-nonylphenol 7 
                 Ethanolamine 
                 Formaldehyde 
               
               
                   
                 mole ethoxylate 
               
               
                 10  
                 4-tert Butylphenol 
                 2-amino-2-ethyl- 
                 Formaldehyde 
               
               
                   
                 1 mole ethoxylate 
                 propane-diol 
               
               
                   
               
            
           
         
       
     
     Example 11 
     This example illustrates how the preferred addition agents may be made using acid catalysis. 
     Formaldehyde (2 moles) was slowly added to ethanolamine (2 moles) with stirring, the temperature being kept to below 60° C. Water (108 g) was added and the mixture acidified to pH 3 using sulphuric acid. The mixture was heated to 60° C. and p-cresol (1 mole) added, on completion of the addition the mixture was heated to 100° C. for 30 mins and then cooled. 
     The electroplating characteristics of various compositions were determined in a Hull Cell at 3 amps total current for 1 minute at 50° C. A 10 cm×6 cm steel plate, pre-cleaned by immersion in sodium hydroxide followed by a water rinse and immersion in 18.5% hydrochloric acid, was used in all the following examples. 
     The aqueous compositions used are out in Table 2, Examples 12-22 are according to the invention whilst Examples 23 and 24 are provided for comparison. 
     In examples 12 and 14 to 21 the composition included 1 g/l of antioxidant (1,2-dihydroxybenzene-4-sulphonic acid). The tin source in all cases was tin sulphate in an amount of 30 g/l calculated as Sn ++ . 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Addition 
                 Addition 
                   
               
               
                   
                   
                   
                 Agent 
                 Agent 
                 Plating 
               
               
                 Example 
                   
                 Acid conc. 
                 (as per 
                 conc. 
                 Range 
               
               
                 No. 
                 Acid 
                 (g/l) 
                 example) 
                 (g/l) 
                 (A/dm 2 ) × 10 
               
               
                   
               
             
            
               
                 12 
                 Sulphuric 
                 60 
                 (4) 
                 6 
                  15-67 
               
               
                 13 
                 PSA 
                 60 
                 (5) 
                 6 
                 5.4-90 
               
               
                 14 
                 PTSA/Sulphamic 
                 10/50 
                 9 parts (2) + 
                 6 
                 6.5-90 
               
               
                   
                   
                   
                 1 part (3) 
               
               
                 15 
                 PTSA/Sulphuric 
                 40/50 
                 (11) 
                 6 
                 5.4-37 
               
               
                 16 
                 Benzenesulphonic 
                 60 
                 1 part (2) + 
                 6 
                  10-55 
               
               
                   
                   
                   
                 1 part (6) 
               
               
                 17 
                 Sulphuric 
                 50 
                 (8) 
                 6 
                  11-82 
               
               
                 18 
                 Methanesulphonic 
                 60 
                 (7) 
                 6 
                 5.4-38 
               
               
                 19 
                 Methanesulphonic 
                 50 
                 (9) 
                 3 
                   6-58 
               
               
                 20 
                 PTSA/Sulphuric 
                 10/40 
                 8 parts (3) + 
                 5 
                 5.4-70 
               
               
                   
                   
                   
                 2 parts (9) 
               
               
                 21 
                 PTSA/Sulphuric 
                 10/50 
                 9 parts (2) + 
                 6 
                 5.4-90 
               
               
                   
                   
                   
                 1 part (9) 
               
               
                 22 
                 PSA 
                 40 
                 (10) 
                 6 
                  11-74 
               
               
                 23 
                 PSA 
                 60 
                 ENSA 6 
                 4 
                   9-54 
               
               
                 24 
                 PSA 
                 60 
                 Diphane V 
                 6 
                   7-52 
               
               
                   
               
            
           
         
       
     
     All concentrations are in g/l of the composition including the water. 
     PSA is phenol sulphonic acid. 
     PTSA is para toluene sulphonic acid.