Patent Application
20250075328
The invention relates to the metallization of polymer surfaces. More specifically, the invention discloses a metallization method with a special adhesion pre-treatment of non-conductive polymer surfaces before metallization thereof, to obtain decorative or functional metal coatings on polymer parts, for their application in various industries.
Metallized polymer products, due to specific physical and chemical properties and price efficiency, are widely used, replacing various metal products. In these industry areas, a typical application is where decorative and/or functional metal coatings on polymer surfaces are required. However, the deposition of a high-quality metal coating, which would be strongly adherent to non-conductive polymer material, requires special innovative metallization and polymer pre-treatment technologies. Generally, there is known a wet method of pre-treatment of the polymer substrate surface, the method known as surface etching. During the procedure, a sufficient surface roughening but without severe damage to the surface, must be ensured. In addition, specific chemical compositions should be applied to create the required chemical bonds on the roughened surface.
During the last decades, a number of environmentally friendly etching technologies have been suggested instead of previously and even up to day used the toxic chromium compounds (US patents U.S. Pat. Nos. 3,445,350, 4,610,895). Although these formulations generally provide rather strong and acceptable etching results, they all have essential drawbacks, however, environmental and human-friendly alternatives are increasingly in demand. Usage of chrome-free etching technologies, such as permanganate or Mn III-based baths (US patent documents U.S. Pat. No. 10,920,321, US2005/0199587) allows the elimination of toxic chromium compounds, but to obtain the required adhesion quality, they all should be operated at high temperatures that leads to the fast degradation of the active component and also an additional equipment for the control of evaporated water from the bath is required. Additionally, a number of the previous or/and subsequent pre-treatment steps are required to prepare the polymer surface for reliable adhesion with metal coating.
As an alternative to Cr(VI), Mn-based etching compositions are suggested (European patent EP2025708, US patent documents U.S. Pat. Nos. 9,023,228, 9,752,074, US2005/0199587). The common disadvantage of all these methods is permanganate ions' fast decomposition in the presence of strong inorganic acids, such as sulphuric acid mainly used in an acidic etching solution, so it would be beneficial not to use a higher than 45% vol. acid concentration in water. Moreover, the higher operation temperatures can even fasten the permanganate degradation, while at lower temperatures, the required adhesion quality is obtained only in case of the extended immersion time.
Various prototype processes for metalizing polymer surfaces are described in patent documents EP4089201, U.S. Pat. Nos. 10,174,250, 10,377,947, and 10,526,709, where acidic permanganate etching solution stabilized with copper sulphate or copper nitrate is used. In both cases, the adhesion pre-treatment steps consist of the following several steps: polymer samples (EP4089201) are pre-etched in EDGA, followed by etching at 70° C. for 10-15 min. Whereas in U.S. Pat. No. 10,526,709, the sample is pre-treated in an alkaline permanganate solution before etching bath. The acidic permanganate etching solution comprises the copper nitrate and before metallization, is followed with the surface activation procedure in the bath containing 0.2 M of cobalt ions. In contrast, the proposed invention enables obtaining the high-level metal/polymer adhesion while utilizing the only one-step etching bath and, that is very important, at room temperature for 5-20 min, i.e., without the need for the first pre-treatment in the alkaline permanganate. Moreover, the presence in the etching composition of two components, namely, both copper and ammonium ions, resulted beneficially as well as in unexpected promoting effect for the further metallization, as far as allows to utilize the activation cobalt bath with a concentration as low as 0.035 M of cobalt ions for successful metallization. It means the presence of both copper and ammonium ions in the proposed technology resulted not solely in the permanganate bath stabilizing effect but also had an obvious promoting effect on the further metallization process, probably due to the formation of strong chemical metal coating/polymer bonds.
Technical problem. The technical problem by the present invention relates to improving metallization of polymer products and technological processes thereof. The invention aims to deposit a strongly adhered metal coating on electrically non-conductive polymer articles with an environment and human-friendly process that would be cost- and quality-effective and easy to maintain. The invention also aims to find solutions for the treatment of electrically non-conductive polymer surfaces, which would be non-toxic and provide sufficient adhesion strength of the metal layers deposited on the polymer surface.
Solution. The present invention discloses a simple, cost-effective, and easy-to-maintain one-step adhesion pre-treatment process before the metallization of non-conductive polymer surfaces. The polymers are but not limited to: polyimides (PI), acrylonitrile-butadiene-styrene copolymer (ABS), and polypropylene (PP), that being also free of toxic substances, especially of hexavalent chromium, and to obtain decorative or/and functional metal coatings on polymer parts for their application in various industries. The invented polymer surface pre-treatment process comprises, particularly, it consists of—only two pre-treatment steps. The polymer pre-treatment is carried out in the only one adhesion pre-treatment chrome-free etching bath containing permanganate ions, inorganic acids such as phosphoric or/and sulphuric acid, and additionally, two other compounds containing, namely, copper and ammonium ions, and water, is followed with subsequent treatment in cobalt activation bath.
Advantages. The invented pre-treatment of polymer surfaces is a cost-effective and easy-to-maintain as compared with other known art chrome-free etching technologies, such as permanganate or Mn III-based etches to obtain decorative or/and functional metal coatings on polymer parts for their application in various industries. To obtain the required adhesion quality, the all previously known processes should be operated at high temperatures, resulting in fast degradation of the active component and also requiring for additional equipment to control evaporated water from the bath. For the coating/polymer adhesion, the present invention employs only one adhesion pre-treatment bath containing inorganic acids, such as phosphoric acid or/and sulphuric acid, permanganate ions, and additionally, two other compounds containing copper and ammonium ions, and water, followed by subsequent treatment in cobalt activation bath with a concentration of cobalt (II) ions as low as 0.035 M, as well as to prolong significantly the service life of etching bath by utilizing it at room temperature. Additionally, the Co-colloid-activation-bath composition is beneficial to obtain quality-effective metal deposition at a much lower concentration of active Co-compound, as compared with other known Co-activation baths (e.g., disclosed in DE19904665). This makes the present Co-activation-bath preferable, also allowing to meet the EPA or/and OSHA requirements on the usage of Co salts.
The invention provides a surprising double effect of promoting the required level of metal coating to polymer adhesion as well as prolonging the etching bath service life due to being operated at room temperature.
Process. A process of metalizing electrically non-conductive polymer surfaces of polymer products/components involves the following steps:
Examples. A plate of acrylonitrile-butadiene-styrene copolymer (ABS) and polyimide (PI) (GoodFelow 0.125 mm Kapton HN) polymer with a geometric area of 8 cm2 was fastened to a copper wire. The sample was rinsed with water containing surface-active substances at a temperature of 40° C. for 5 min. After that, the sample was treated in the acidic permanganate etching solution containing 0.06 M KMnO4, 0.4 M copper sulphate and 0.2 M ammonium sulphate, 12.4 M phosphoric acid, and the rest water. The etching is performed at room temperature for 5-20 minutes. After the etching step, the sample is washed with water and then activated in the 0.035 M cobalt colloidal solution (pH is about 6) at room temperature for 1 min, followed by washing with water and subsequent treatment in 0.064 M Na2S solution for 30 s at room temperature. Then, the sample is washed again with water. To completely cover the polymer surface with electroplated nickel, two cycles of sample activation are required; otherwise insufficiently active surface is not formed.
It should be noted that this cobalt colloidal solution is stabilized with NH4CH3COOH, Na2B4O7*10H2O, and chelating agent selected from the group of polyhydroxylic compounds having 2-6 hydroxyl groups) and remains active for a long time at room temperature. Moreover, the cobalt activation bath acquires its maximum activity after a day of storage at room temperature or after heating to 40 degrees for 10-20 minutes.
The resulting solution is of burgundy color, and stable for a long time. Without chelating agent, the solution is active, green in color, but stable for a shorter time. After a month of aging at room temperature, sediment appears on the bottom of the solution.
After the adhesional pre-treatment, the polymer sample is coated with electrochemical nickel coating using the Ni plating bath contained (g/l): NiSO4*7H2O—150 g/l; NiCl2—30 g/l; H3BO3—30 g/l. The bath operated at pH 4. To measure the adhesion of electrolytically deposited Ni coating, it was further thickened with an electrochemically plated copper coating with a thickness of approximately 50 μm using the Cu plating bath contained (g/l): CuSO4*5H2O—150 g/l; H2SO4—96 g/l. To evaluate Ni coating's adhesion to the polymer surface, the force needed to peel a 1 cm wide strip of the coating at a 90-degree angle is measured.
The resulting adhesion strengths of a nickel coating deposited after the polymer activation in a cobalt solution bath containing different concentrations of chelating agent are given in Table 1. Meanwhile, a larger amount of chelating agent in the activation solution reduces the solution activity, i.e., coating adhesion decreases by 10-15% (see Table 1, examples 2 and 3). Freshly made Co colloidal solution (Co activation bath) (but already acquired) activity, compared to solutions kept at room temperature for a month, did not significantly affect the adhesion values (Table 1).
Conditions of the polymer's pre-treatments and the obtained values of Ni coating adhesion are given in Table 2. It was observed that when measuring the adhesion of freshly deposited metal coatings on ABS, it is almost 3 times lower compared with the adhesion of coatings that have been dried for at least a week at room temperature. (Table 2, examples 1 and 2). For ABS, just 5 minutes in phosphoric acid etching solution containing ammonium ions is sufficient. After etching for 20 minutes, ABS coating retains its gloss and good adhesion, and there is a low risk of over-etching (Table 2).
Another group of substrate samples was prepared using the etching solution with the addition of a low amount of sulphuric acid (1.8 M) together with phosphoric acid (11 M). Notably, the addition of a small amount of sulphuric acid to the etching solution did not affect the adhesion of the coating deposited on the polymer, but during the etching, it was observed that the surface of the polymer sample became hydrophilic earlier than in the absence of sulphuric acid, so the etching time for ABS could be less than 5 minutes.
PI samples prepared in the same way are required twice longer: 10, but not 5 min. It was not possible to tear off the metal coating from the PI film because it tears together with the polymer, i.e., the adhesion of the coating is stronger than the tearing force of the polymer.
