Golden and shining colors were used thousands of years ago to provide added value to objects such as buildings or statues. Until the end of the 19th century, gold leaf was applied for these purposes, but the gilding process was very time-consuming and the raw material extremely expensive. At the beginning of the last century, owing to new manufacturing techniques, gold leaf was gradually replaced with metal-effect pigments based on copper zinc alloys. These gold bronze pigments have found widespread use in the printing, coatings and plastics industries whenever brilliance and metallic golden luster are required.
Manufacturing and PropertiesGold bronze pigments are made from cathodic copper and pure zinc. These metals are alloyed, atomized to small particles and ground to flakes in ball mills. Depending on the desired particle size, up to five grinding steps are necessary. Stearic acid is added as a lubricant to prevent cold-welding of the pigments. After a subsequent polishing and a final classifying step, the mill charges are homogenized and the resulting standardized pigment material is filled into drums (Figure 1).
Owing to the position of copper and zinc in the electrochemical series, different oxidation reactions can occur. Copper reacts with oxygen, especially at elevated temperatures, and black copper(II)oxide is formed (tarnishing). This oxide layer is soluble in water in the presence of acids or metal complex-forming compounds (e.g. amines, halides), and so provides no protection against further corrosion processes.
Zinc metal can be oxidized easily due to its electronegative character. Acidic as well as strong alkaline conditions lead to water-soluble Zn(II)-species.
These chemical reactions may take place in different coatings, printing and plastics application systems and may cause undesirable effects. Unprotected bronze pigments can tarnish, e.g. during the curing process of powder coating or during the extrusion process of thermoplastics. The resulting products lose their attractive metallic appearance. In both water- or solventborne liquid coating formulations, copper ions can be released and may then cause gelling effects (e.g. in nitrocellulose), or discoloration of binder systems (greening).
To reduce these undesirable effects two different approaches are possible.
1. Chemical inhibition via additives: special corrosion inhibitors are able to adsorb onto the metal surface and limit further attack and/or trap free metal ions, thus preventing their reactions with binder components.
2. Encapsulation of the pigment with a chemically inert and highly transparent barrier layer.
Encapsulation of Gold Bronze Pigments with SilicaThe encapsulation of pigments with silica is well known. Silica-coated gold bronze pigments have been available for many years. Until recently, because of the manufacturing process, they have had less brilliance, color intensity and hiding power compared to uncoated pigments. These drawbacks have now been overcome using a new production technique (Figure 4).
Bronze flakes that have already been processed for color and particle size distribution are dispersed in a defined mixture of ethanol, water and the silica precursor tetraethoxysilane. The quantity of silane depends on the particle size of the pigment. A catalyst is added and the whole mixture is heated up to accelerate the reaction. The silane is hydrolyzed and reacts to form silica, which precipitates onto the surface of the metallic flake in the form of an ultra-thin vitreous layer (sol gel process).
When the reaction is completed, the mixture is filtered to remove undesirable by-products such as copper compounds, released lubricants (stearic acid) and the catalyst. In the final drying step, the residual solvent is evaporated and the coated gold bronze pigments are obtained in the form of a free flowing powder.
The silica layer itself can be altered by the reaction, for example, with bifunctional surface modifiers1, or other surfactants, leading to metal pigments with tailor-made wetting properties.
Properties of Silica-Coated Gold Bronze PigmentsThe silica, approximately 3 to 5% of the metal flake weight, builds up a homogenous and highly transparent, but extremely thin, layer. The image from the scanning electron microscope, (Figure 5), shows the smoothness and homogeneity of the pigment surface, which has no silica side precipitations.
At present these pigments are available in eight different shades (natural as well as oxidized), and in a wide range of particle sizes (Table 2). The coarse grades give coatings a colorful metallic sparkle, whereas the finer grades provide excellent coverage. As indicated above, the amount of the silica depends on the particle size and, hence, the specific surface of the starting material. This is also the reason why very fine grades (D50 < 8 æm) are not yet commercially available.
Bronze pigments are normally very sensitive to shear stress, owing to their shape (flakes) and the ductile nature of the alloy. The silica layer not only improves the thermal stability but also significantly enhances the resistance against mechanical damage. This has been proved using the high shear "Waring Blender Test". Both a coated and uncoated pigment grade, (pale gold, D50=17 æm), were mixed in a high-solid system (8 minutes, 13,500 rpm), followed by an optical characterization of the sheared vs. un-sheared coatings. According to the test procedure, when measured at 15¡, a DL* value < 5 is considered non-degrading, 5-10 semi-degrading and >10 degrading. The silica-coated bronze pigments passed the test with a DL* = -2.0. The uncoated pigments not only showed strong mechanical damage, (DL* = -20.2), but also were strongly corroded during the 1-day storage that follows the shear test (greening of the binder).
The barrier effect of the silica layer against thermal influences makes them non-flammable regardless of the particle size (tested according to Dir. 92/69/ECC). Consequently, these gold bronze pigments are not classified as "dangerous goods" for transportation. Additionally, samples of silica-coated pigments were investigated with respect to human health. Neither skin-patch tests nor in-vitro toxicity tests gave any indication of dermal or ophthalmic irritation, or allergic contact sensitization.
Semi-Leafing Gold Bronze Pigments for Powder CoatingsSilica-coated gold bronze pigments, due to their good wettability and non-leafing character, become uniformly distributed throughout the powder coating film and do not orientate parallel to the surface. Consequently, the coating has high gloss values (depending on the powder base), but relatively low color strength and metallic brilliance.
Silica-Coated Gold Bronze Pigments for Direct Extrusion Into Powder CoatingsAs described above, it is absolutely necessary to protect gold bronze pigments with a silica layer, against the curing temperature used in powder coating applications. Up to now it has not been possible to process state-of-the-art gold bronze pigments into powder coatings by direct extrusion. Their flake shape and shear-sensitive nature cause them to be damaged, especially during the grinding stage. In the following powder coating application process, the partly chopped and bent pigments did not orientate properly, and the appearance was merely dark brown, without any metallic effect. Metallic powder coatings, therefore, had to be produced by either a dry-blending process or an additional operation, bonding.
In the bonding process, an agitated mixture of base powder and effect pigments is gently warmed up above the softening point of the resin. The pigments are dispersed and stick evenly to the base particles. This method provides the best results with respect to optics and recovering of the oversprayed powder. Dry blending is cost saving, but as the "heavy" bronze pigment can separate from the base during spraying, the recovered and the sprayed powder tend to change in composition.
Recently, a new generation of metal-effect pigments was created to overcome these drawbacks. The basic bronze pigments are similar to the above-mentioned 17-æm grades, and the silica provides sufficient mechanical and thermal stability.
In Figure 9, a comparison between a silica-coated metal pigment (e.g. Dorolan 17/0 Copper) and the new pelletized pigment preparation (e.g. PowderSafe 1790-01 Copper) is given. Before spraying, both pigments were processed via an extruder and a milling unit.
As the pigments orientate close to the surface of the coating, their corrosion protection through the binder is rather limited. For outdoor stability and whenever high chemical resistance of the finished powder coat is required, a UV-stable topcoat is still recommended. Stability has to be checked in customers' specific coating system to ensure that the product meets all application requirements.
Conclusion and Further OutlookA new generation of gold bronze pigments, silica coated by a sol-gel process, was presented. This silica layer provides a short- to medium-term stability sufficient for many applications.
The results of our work give guidance on how to achieve brilliant metallic effects in the reddish to greenish-gold color range. The properties of coatings always result from the interaction between the pigments and all the other components. The possible variations are vast.
We, therefore, consider it unlikely that one kind of (gold bronze) pigment will satisfy all optical and stability requirements of the potential coating systems you may formulate. This fact however, creates the opportunity for further developments to satisfy more specific requirements as they arise.
AcknowledgmentsI would like to thank my lab team (Mrs. Kupfer, Mrs. Meerstein and Mrs. Pickelmann) for most of the preparative work, and the colleagues from Technical Service Department (Mr. Korn, Mr. Schreiber and Mr. Wissling) for testing the new products. We are grateful to Klaus Greiwe, Ulrich-Andreas Hirth and Robert Lewis for valuable discussions and contributions.
For more information, contact Dr. Hans-Joerg Kremitzl, Eckart GmbH & Co. KG, Plant Guentersthal, D-91235, Velden, Germany; phone +49 9152 774850; or e-mail firstname.lastname@example.org.
This paper was presented at the 7th Nurnberg Congress, European Coatings Show, April 2003, Nurnberg, Germany.