Abstract:
A method for depositing bright zinc from zinc electrodeposition baths in which a compound prepared by reacting imidazole and/or at least one imidazole derivative with at least one organic compound which quaternizes nitrogen in said imidazole in the presence of water is added to the alkaline zinc electroplating baths.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for brightening the electrodeposits of zinc from alkaline zinc electroplating baths. 
     2. Description of the Prior Art 
     Various types of brightener additives for alkaline zinc electroplating process such as an organic aldehyde compound combining with a substituent, a ketone compound, a heterocyclic compound, a sulfur organic compound and/or a water-soluble high molecular compound have been widely investigated. These brightener additives which have been developed in the prior art are not satisfactory for use in a specific alkaline zinc electroplating bath such as the zincate or zinc cyanide baths, so far as workability and the plated zinc at the high current density are concerned. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to a method for brightening by employing an additive which is well suited for workability at high current density and does not give &#34;burnt&#34; deposit or pits on the plated zinc. 
     Briefly, this object and other objects of the invention as hereinafter will become more readily apparent can be attained by providing a brightener additive for alkaline zinc electroplating baths which comprises a compound prepared by reacting imidazole and/or at least one imidazole derivative with at least one organic compound which quaternizes nitrogen in said imidazole in the presence of water. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The starting materials for the preparation of the brightener of this invention include imidazole and/or derivatives thereof such as 1-methylimidazole, 1-ethylimidazole, 2-methylimidazole, 1,5-dimethylimidazole, 1-ethyl-2-methylimidazole, 1-oxymethylimidazole or 1-vinyl imidazole and include the quaternizing agents such as monochloroacetic acid, benzyl chloride, chloroacetoamide, 3-aminobenzyl chloride, dichloroglycerine, methyl iodide, allyl chloride, dichloroethane and/or monochloropropane. 
     The brightener additives can be prepared by adding at least one quaternizing agent to imidazole and/or at least one derivative thereof in quantities ranging from one mole to 1.5 moles per mole of the imidazole and then reacting the materials for from 0.5 to 4 hours at from 40° to 100° C. in the presence of such water quantities that the reaction products are dissolved. The brightener additives thus prepared are added to the zincate electroplating bath in ratios of 0.1-3 g/l, or the zinc cyanide electroplating bath in ratios of 0.05-0.5 g/l. 
     The plated zinc of this invention once electrodeposited exhibits a considerably enhanced brightening effect and are satisfactory for use at high current density and moreover any &#34;burnt&#34; deposit or any pits are not found on the plated zinc. 
     The brightener additives of the prior art hereinafter disclosed can be added to the brightener additives of this invention, if desired. 
    
    
     Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified. The Examples show the preparation of some of the brightener additives of the present invention and a bright zinc layer electrodeposited by each bath used containing one of the brighteners. 
     EXAMPLES 
     Imidazole, 1-ethylimidazole, benzylchloride, monochloroacetic acid and water were added to the following mixing ratios to a four-necked flask equipped with a thermometer, a condenser, a stirrer and a separatory funnel. 
     
         ______________________________________Mixing ratios______________________________________No.1       Imidazole         6.8    g      Benzyl chloride   12.6   g      Water             10.0   gNo.2       Imidazole         6.8    g      Monochloroacetic acid                        9.5    g      Water             10.0   gNo.3       1-ethylimidazole  8.2    g      Benzyl chloride   12.6   g      Water             10.0   g______________________________________ 
    
     The mixture was warmed to 80° C. for 1 hour. The reaction products thus prepared were diluted to 100 g with water. 
     The above water solutions were added at the various ratios to the zincate electroplating bath and the zinc cyanide electroplating bath of which the compositions are shown as follows: 
     
         ______________________________________Bath of medium cyanide concentration (M-CN)  Zn            20 g/l  NaCN          40 g/l  NaOH          80 g/lBath of low cyanide concentration (L-CN)  Zn            10 g/l  NaCN          12.5 g/l  NaOH          70 g/lZincate bath (Z)  Zn            10 g/l  NaOH          120 g/l______________________________________ 
    
     The electroplating of zinc on steel was performed by passing an electric current at a bath temperature of 25° C. 
     The experimental results are shown in the following table. 
     
                                           Table__________________________________________________________________________Test Number  1   2   3   4   5   6   7   8   9   10  11Kind of Bath Z                           L-CN    M-CN            Com-        Com-    Com-    Com-    Com-            para-       para-   para-   para-   para-            tive        tive    tive    tive    tive        No.1            Ex. No.2    Ex. No.3                                Ex. No.1                                        Ex. No.2                                                Ex.__________________________________________________________________________Kind ofThis Invention        1   --  0.8 0.8 --  2   --  0.1 --  0.05                                                -- (g/l)Bright-A (g/l) 4   4   --  --  --  2   2   0.5 0.5 --  --ner  B (g/l) --  --  6   6   6   3   3   --  --  --  --Addi-Anisaldehyde (g/l)  0.5 0.5 --  0.5 --  0.5 0.5 0.2 0.2 0.1 0.1tive Polyvinylaldehyde (g/l)        --  --  --  --  --  0.2 0.2 0.2 0.2 0.2 0.2Current Density        0.5-6            0.5-2                0.5-7                    0.5-7                        0.5-2                            0.5-8                                0.5-2.5                                    0.3-8                                        0.3-3                                            0.3-8                                                0.3-4Brightness of        Very        Very                        Fairly                            Very    Very    Veryplated Zink  Good            Good                Good                    Good                        Good                            Good                                Good                                    Good                                        Good                                            Good                                                GoodBurntdeposit or              Found           Found   FoundPits on Plated        Not     Not Not in  Not     Not in a                                            Not in aZink (Current        Found            Found                Found                    Found                        Large                            Found                                Found                                    Found                                        little                                            Found                                                littleDensity 4 A/dm.sup.2)        Number          Number  Number__________________________________________________________________________ 
    
     The comparative examples in which two kinds (A and B) of brightener additives in the prior art were added to the same alkaline electroplating baths in place of the present brightener additives are shown together in the table. The brightener additive A is a 50% water solution of the reaction product of methylamine and epichlorohydrin, and the brightener additive B is a 50% water solution of polyethyleneimine (polymerization degree, about 2000). 
     Moreover, though the cases in which the brightener additive No. 2 or No. 3 was added in the electroplating bath L-CN, and the cases in which the brightener additive No. 1 or No. 3 was added in the electroplating bath M-CN were not shown in the above table, the same superior results as could be seen from the cases of the electroplating bath Z were obtained in bath cases. 
     Having now fully described this invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made there to without departing from the spirits or scope of the invention as set forth herein.