Abstract:
Pulse reverse electrolysis of acid copper solutions is used for applying copper to printing cylinders, especially gravure printing cylinders. The plating composition generally comprising copper ions, counter ions, chloride ions, a polyalkylene glycol, and a bath-soluble divalent sulfur compound. The benefits include an improved thickness distribution of the copper electrodeposited on the plated article, reduced metal waste, reduced plating times and increased production capacity.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a method for manufacturing gravure printing cylinders.  
       BACKGROUND OF THE INVENTION  
       [0002]     The plating of copper from acid solutions is well known, with numerous industrial applications. In most applications the articles to be plated are suspended in the electrolyte, a technique generally referred to as rack plating. Rack plating is a well-known process and examples of the process may be found in U.S. Pat. No. 3,939,056 to Fueki et al., U.S. Pat. No. 4,176,039 to Wismer, and U.S. Pat. No. 4,297,197 to Salman, the subject matter of each of which is herein incorporated by reference in its entirety.  
         [0003]     Another application well known to those in the industry is copper plating steel and aluminium cylinders used for printing applications, such as gravure printing, wherein steel or aluminium is used to provide a cheap and sturdy substrate for the engraveable copper coating. Gravure printing is a method using the Intaglio process in which the image to be printed consists of depressions etched or engraved usually to different depths. Slightly viscous solvent inks are applied to the entire surface and a metal doctor blade removes the excess ink from the non-printing surface. Normally engraving is performed on a copper plated cylinder, which is subsequently chrome plated to minimize wear.  
         [0004]     The electroplating and other associated treatment of the printing cylinders normally takes place in a suitable tank containing an electrolyte into which the cylinder is partially or wholly immersed. The cylinder is made the cathode and a direct electric current is passed through the cylinder and electrolyte with suitable anodes completing the circuit with a power supply. The cylinder is rotated during the process and the tanks are normally fitted with filtration and temperature control equipment to provide good process control. If desired, further solution agitation equipment such as air or solution movement may be utilized.  
         [0005]     Printing cylinders are prepared for acid copper plating by first cleaning the surface to remove oils, dirt and grease and surface oxidation products, thereby providing a suitable surface for electroplating. Steel cylinders are then coated in a thin layer of copper from a solution of copper (I) ions, normally from a cyanide-based electrolyte. This ensures an adherent copper deposit by avoiding the displacement reaction experienced with copper (II) electrolytes on steel substrates that may lead to poorly adherent deposits. Aluminium cylinders are processed through a chemical pre-treatment that produces a thin zinc layer on the aluminium, which then renders the substrate suitable for applying a thin layer of copper or nickel from solutions of near neutral pH.  
         [0006]     The cylinders with a thin copper or nickel deposit are then copper plated from an acidic copper (II) electrolyte to build up a thick layer of copper, normally in the order of 100 to 500 microns. Acidic copper electrolytes are preferred to copper (I) electrolytes for the second stage because of their ability to plate faster by the use of high current densities. Typically current densities of 20 to 25 A/dm 2  are employed. Examples of prior art processes for copper plating of gravure printing cylinders are described in U.S. Pat. Nos. 5,417,841 and 4,781,801 to Frisby, the subject matter of each of which is herein incorporated by reference in its entirety.  
         [0007]     Due to the nature of the current distribution during the plating process, the copper deposit on the printing cylinder can be very uneven and the ends of the cylinders generally tend to have a much thicker deposit than the center of the cylinder. The degree of this variation varies according to the cylinder dimensions and the plating conditions, but it is not uncommon for a printing cylinder to have a copper deposit where the thickness at the cylinder ends is twice the thickness as the center of the cylinder, which makes the cylinder unsuitable for the printing process. To make the cylinder suitable for printing, the copper plated cylinder must be treated to provide a surface that has a deposit, for example, that does not vary more than about +/−2 microns across the cylinder surface. This is normally achieved by two polishing operations; first, a machining operation that removes relatively large amounts of copper, and secondly a fine polishing operation.  
         [0008]     Cylinders that have been electroplated, machined and polished as described above are then ready to be engraved or etched with the design to be printed. For accurate and consistent engraving it is necessary that the copper deposit be of a suitable uniform hardness, which can be controlled by additives in the copper plating solution. It is also necessary that the copper deposit is free from deposit imperfections such as voids (pits) and roughness. Once engraved or etched with the print design, the cylinders are electroplated with a thin layer of chromium to provide wear resistance during the printing operation.  
         [0009]     During the printing operation, wear of the cylinder takes place and worn cylinders are typically reconditioned by removing a predetermined thickness of deposit by a further machining stage, such that the entire print design is removed from the cylinder surface. The thickness removed is normally in the order of about 100 microns. The cylinder can then be made suitable for further electroplating of copper to return the deposit to the original plated thickness. Subsequently, the cylinder is subjected to the normal machining, polishing and engraving process and made useable for further printing. In this way a single cylinder can be used continually.  
         [0010]     The drawback of the current technique for preparing cylinders is that in order to achieve the desired thickness across the entire length of the printing cylinder, a minimum thickness of copper must be deposited in the center of the cylinder. The excess is then removed from the edges by machining, which is a costly and time-consuming operation that generates a large amount of waste copper metal. Thus, there remains a need in the art for an improved process for preparing gravure printing cylinders that does not require a subsequent machining step to achieve an even deposit of copper across the length of the printing cylinder.  
         [0011]     The use of pulse reverse plating techniques to deposit copper from acidic solutions is well-known within the electronics industry, for plating copper from acidic solutions onto printed circuit boards and other substrates. U.S. Pat. No. 6,319,384, to Taylor et al., the subject matter of which is herein incorporated by reference in its entirely, discloses a method for the electrodeposition of copper onto a semiconductor substrate, wherein the acidic copper plating bath is substantially devoid of brighteners and and/or levellers.  
         [0012]     The basic chemistry of the additives used for electronics applications, and their performance under pulse current plating conditions as compared to direct current conditions is explained by T. Pearson, “Effect of Pulsed Current On The Electrodeposition of Chromium and Copper”, PhD thesis, Aston University, United Kingdom, 1989. The additives are broadly similar to those used in general rack plating applications, and broadly comprise a sulfopropyl sulfide and a polyalkylene glycol that operate in conjunction with chloride ion. These holes are typically 0.5 mm diameter and 2-3 mm deep. Typically the cathodic current density used in the plating of printed circuit boards is in the order of 2.0 A/dm 2 , but due to the geometrical factors the effective current density in the holes is extremely low and much lower than the average current density applied to the board as a whole.  
         [0013]     U.S. Patent application publication No. 2004/0074775 to Herdman et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of pulse reverse plating and acid copper electrolytes for decorative copper applications such as plating on plastics for automobile or sanitary applications, or plating on alloy automobile wheels. The process provides for improved distribution of the copper deposit across the substrate. The current density range described in the above application is 0.5-5.0 A/dm 2  but typically the applied current density is 2.0-3.0 A/dm 2 . However, this current density range is about an order of magnitude below the current densities require for the copper plating of printing cylinders.  
         [0014]     A further recent U.S. Patent application publication No. 2002/0079228 to Smith, the subject matter of which is herein incorporated by reference in its entirety, describes an apparatus and method for electroplating of gravure printing cylinders. The method employs the application of pulse reverse plating to a bath based upon copper sulfate, sulfuric acid and chloride ion.  
         [0015]     The inventors have surprisingly found that the pulse reverse current plating techniques used for printed circuit boards can also translate very well to the application of plating copper in the conditions used for plating printing cylinders. This is surprising in that the current density range is very different from that applied during plating of printed circuit boards or during rack plating applications, and also surprising because in normal electroplating applications an increased current density results in a worse metal distribution. When printing cylinders are plated using pulse reverse plating in conjunction with an additive system optimized for pulse reverse electrolysis, the distribution of copper deposit across the length of the cylinder is dramatically improved. This has two distinct advantages to the plater: (1) a reduction in excess copper deposited on the ends of the cylinder, and (2) the central area of the cylinder can be plated to the required thickness in a reduced time or at a reduced current density. The advantages described above can lead to reduced polishing requirements, reduced scrap metal production, increased production capacity and energy savings. The deposits produced are levelled, free from surface imperfections and hardness control can still be achieved by certain bath additives.  
       SUMMARY OF THE INVENTION  
       [0016]     The use of pulse reverse plating to deposit copper can be used for a method of plating printing cylinders in an acidic copper electroplating bath comprising the steps of: 
        (a) suspending the cylinder in a plating bath comprising copper ions, counter ions, chloride ions, a polyalkyleneglycol and a bath-soluble divalent sulfur compound; and     (b) plating the cylinder for a sufficient period of time with alternating cathodic and anodic current to produce a desired thickness of copper on the surface of said cylinder.       
 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     The present invention utilizes pulse-reverse current for plating gravure printing cylinders with copper in an acidic copper plating bath to produce a desired thickness of copper on the surfaces of the cylinders. The present invention is particularly useful for plating a uniform thickness of copper across the length of the printing cylinder.  
         [0020]     The acidic copper plating bath of the invention generally comprises copper ions, a source of counter ions, chloride ions, a polyalkylene glycol, and a bath-soluble divalent sulfur compound. Other additives such as wetting agents may also be added to the bath to improve the copper deposit.  
         [0021]     Copper ions are present in the plating bath at a concentration of about 30 to 70 g/l. Copper sulfate pentahydrate is an example of a copper compound that is useful in the baths of the present invention. Other copper compounds known to those skilled in the art, including as copper methanesulfonate, and mixtures of such compounds, are also suitable. The plating bath generally comprises the copper sulfate pentahydrate at a concentration of about 120 to 280 g/l, preferably about 150-200 g/l.  
         [0022]     The source of counter ions in the plating bath is most commonly sulfate ions, but may also be methanesulfonate ions or a mixture of such ions. A preferred source of sulfate ions is sulfuric acid. Where sulfate is the counter ion, sulfuric acid is normally present in the plating bath at a concentration of about 50-250 g/l, preferably about 80-140 g/l, and most preferably about 100-110 g/l.  
         [0023]     Chloride ions may also be present in the plating bath, at a concentration of about 10-500 mg/l, preferably about 75-150 mg/l. The source of chloride ions in the plating bath is preferably hydrochloric acid.  
         [0024]     The polyalkyleneglycol is generally present in the plating bath at a concentration of between about 50 and 10,000 mg/l, preferably between about 300 and 1,000 mg/l. The polyalkyleneglycol typically has a molecular weight of between 500 and 100,000. Preferred polyalkyleneglycols include polyethylene glycol and an ethylene oxide/propylene oxide co-polymers. A mixture of such suitable polyalkyleneglycols may also be used.  
         [0025]     The bath-soluble divalent sulfur compound is generally present in the plating bath at a concentration of about 1-150 mg/l, preferably about 30-50 mg/l. Preferred divalent sulfur compounds include, but are not limited to, mercaptopropanesulfonic acid or an alkali metal salt thereof, bis-(propane-3-sulfonic acid) disulfide or an alkali metal salt thereof, and bis-(ethane-2-sulfuric acid)disulfide or an alkali metal salt thereof, and mixtures of one or more of the foregoing.  
         [0026]     Other commercially available additives such as wetting agents, brighteners etc. may also be added to the plating bath compositions of the instant invention. The additives may be added to minimize pit formation, or to modify the deposit properties, for example the hardness or the visual appearance. Such additives are generally well known to those skilled in the art.  
         [0027]     The pulse plating regime of the plating bath consists of alternating cathodic and anodic pulses. The cathodic pulse time is generally between 5 and 100 milliseconds, and the anodic pulse time is generally between 0.1 and 10 milliseconds. Optionally, the plating regime may additionally include a cathodic period of extended time, such as up to about 1 hour or may include a short period, for example between 0 and 10 milliseconds, of zero current between the anodic and cathodic pulse, generally referred to as “dead time”.  
         [0028]     The printing cylinders may be completely or partially immersed in the copper plating bath composition of the invention. Preferably, the printing cylinders are partially immersed in the copper plating bath. In addition, the printing cylinders may be rotated in the plating bath composition.  
         [0029]     The average applied current density is generally between about 10.0 and 35.0 A/dm 2 . The current density during the anodic pulse is typically between 1 and 5 times the current density during the cathodic pulse.  
         [0030]     In an optional, but preferred embodiment, a layer of chrome may subsequently be applied over the layer of copper on the printing cylinder. This layer is typically applied by means of electroplating.  
       EXAMPLES  
       [0031]     The following non-limiting examples demonstrate various attributes of the instant invention. In the following examples, a typical printing cylinder having a diameter of diameter 210 mm and a length of 400 mm was electroplated in acid copper solutions. Prior to electroplating, the printing cylinder had previously been copper coated and milled flat.  
         [0032]     The electrolyte temperature during the tests was approximately 30° C. During plating, the cylinder was 50% immersed in the solution and rotated at 75 rpm. It is noted that the current density applied refers to the immersed portion of the cylinder only.  
       Example 1  
     Prior Art  
       [0033]     A bath composition comprising the following was used:  
                                                       Copper sulfate pentahydrate   220 g/l           Sulfuric acid    35 g/l           Chloride ion   none           Proprietary additives   yes           Plating regime:   direct current at 25 A/dm 2             Plating time:    1 hour                      
 
         [0034]     The deposited copper had a bright appearance. No pitting, nodules, or other defects were observed on the surface of the printing cylinder.  
       Example 2  
       [0035]     A bath composition comprising the following was used:  
                                       Copper sulfate pentahydrate   150 g/l       Sulfuric acid   105 g/l       Chloride ion    85 mg/l       Polyethylene glycol (MW 12,000)   400 mg/l       bis-(propane-3-sulfonic acid    35 mg/l       disulfide, disodium salt       Proprietary additives   no       Plating regime:   Pulsed at 15 A/dm 2  average            38 ms forward            2 ms reverse at 2× the forward           current       Plating time:    1 hour                  
 
         [0036]     The deposited copper had a bright appearance. No pitting, nodules, or other defects were observed on the surface of the printing cylinder.  
       Example 3  
       [0037]     A bath composition comprising the following was used:  
                                       Copper sulfate pentahydrate   150 g/l       Sulfuric acid   105 g/l       Chloride ion    85 mg/l       Ethylene oxide/propylene oxide   400 mg/l       co-polymer (MW 12,000)       Mercaptopropanesulfonic acid,    25 mg/l       sodium salt       Proprietary additives   yes       Plating regime:   Pulsed at 15 A/dm 2  average            20 ms forward            1 ms reverse at 2× the forward           current       Plating time:    1 hour                  
 
         [0038]     The deposited copper had a bright appearance. No pitting, nodules, or other defects were observed on the surface of the printing cylinder.  
       Example 4  
       [0039]     A bath composition comprising the following was used:  
                                       Copper sulfate pentahydrate   150 g/l       Sulfuric acid   105 g/l       Chloride ion    85 mg/l       Ethylene oxide/propylene oxide   300 mg/l       co-polymer (MW 33,000)       bis-(ethane-2-sulfuric acid    40 mg/l       disulfide, disodium salt       Proprietary additives   yes       Plating regime:   Pulsed at 15 A/dm 2  average            10 ms forward               0.5 ms reverse at 2× the forward           current       Plating time:    1 hour                  
 
         [0040]     The deposited copper had a bright appearance. No pitting, nodules, or other defects were observed on the surface of the printing cylinder.  
         [0041]     In order to determine the copper thickness, the diameter of the cylinder was measured prior to, and after, the plating period using an accurate micrometer at five points along the cylinder as shown in  FIG. 1 . The increase in diameter after the plating period was divided by two to calculate the deposit thickness in microns. The results of the copper thickness measurements performed for each of the examples is presented in Table 1.  
                                                                   TABLE 1                           Deposit thickness at various points across       the width of the printing cylinder (μm)                Test   Test   Test   Test   Test           spot 1   spot 2   spot 3   spot 4   spot 5                        Example 1   152   97.6   82.3   95.4   149       Example 2   102.5   100   97.5   100   100       Example 3   107.5   106   105   105.5   106       Example 4   103   100   101   99   100                  
 
         [0042]     The deposit hardness was measured using a calibrated commercially available hardness measurement device (Model CuH1, available from Graphische, Technik and Handel Heimann GmbH), at the same points as shown in  FIG. 1 . The results of the deposit hardness measurements at various points across the width of the printing cylinder are presented in Table 2.  
         [0043]     As is readily seen, use of the process of the instant invention, as demonstrated in Examples 2-4 the use of pulse reverse plating with the novel plating compositions of the invention provides a deposit that varies less than +/−2 microns over the surface of the printing cylinder. On the other hand the prior art method using direct current provides a deposit that varies widely over the surface of the printing cylinder. The prior art plating cylinder would necessarily need a further step of polishing, as discussed above in order to be usable as a printing cylinder, requiring additional time and expense over the novel process described by the inventors of the present invention.  
         [0044]     The deposit hardness was measured using a calibrated commercially available hardness measurement device (Model CuH1, available from Graphische, Technik and Handel Heimann GmbH). The results of the measurements are provided in Table 2 for Examples 1 and 4. The results are presented in units of HV300, i.e., a Vickers hardness scale, with a 300 gram load.  
                                                                   TABLE 2                           Hardness at various points across the       width of the printing cylinder (HV50)                Test   Test   Test   Test   Test           spot 1   spot 2   spot 3   spot 4   spot 5                        Example 1   217   212   213   214   212       Example 4   232   232   231   232   229                  
 
         [0045]     While the invention has been described in the context of electroplating gravure printing cylinders, the invention is not limited to this particular application and can be suitably used in other applications requiring similar plating compositions and plating conditions and for similar substrates.