A review on the direct electroplating of polymeric materials

One of the most important groups of materials used in industry, besides metals and ceramics, are polymer materials. The rapid development of their applications results from high resistance to chemical substances, low density, and easy manufacturing of products with complex shapes. The initial research on covering polymeric materials with metal layers can be dated back to the 1950s when the first electroplating baths were developed to enable the application of copper and nickel coatings [1]. Historically, the first metallic coatings exhibited poor adhesion to the polymeric substrates. The discovery of the poly(acrylonitrile–butadiene–styrene) (ABS) terpolymer made it possible to achieve a large increase in the adhesion of metallic coatings. ABS is a thermoplastic material whose basic phase is acrylonitrile-styrene in which the butadiene phase is dispersed. In this material, selective dissolution of the dispersed butadiene phase with organic solvents can take place. The pores formed in this way increase the contact surface between the substrate and the metal layer, increasing the interphase adhesion [2].

Metallization of polymer materials was initially used as a decorative finishing for details, and with the development of this technology, the range of applications was broadened [3]. Nowadays, metalized plastics are used also in electronics as printed circuits, antennas, connectors, as equipment for cars, airplanes, and ships [4, 5]. Metallization of casing surfaces is used in electromagnetic (EM) shielding of electronic devices. A particularly effective method of reducing the level of the electromagnetic field is the use of barriers made of materials characterized by high electrical conductivity. In addition to typical materials (copper, aluminum, brass, steel) used as EM screens, polymer composites, polymer nanocomposites, conductive polymers, or metalized polymers can also be applied [6‐10]. An important area of application of the metallization of polymer materials is the MID (Molded Interconnect Devices) technology. Its technological process can be generally divided into two stages: injection molding and application of conductive tracks and solder pads by selective metallization of the material [11]. MID has found application, among others, in the rapid prototyping technology, which allows the production of products directly based on CAD data, without the need for mechanical processing. Such a product can be molded from a thermoplastic material in additive manufacturing (2D/3D). Built-in MID, 2D / 3D technology, switches, resistors, capacitors, printed circuits, etc., can be electrically connected in various geometric configurations. This allows electronic components to be installed in free spaces of devices, for example inside a car lamp, cockpit, or other covered technical product. This solution may reduce the use of structured wiring and reduce control units.

This work is an attempt to find and analyze available information on the metallization of polymers and their composites, with emphasis on electroplating as a method that pollutes the environment to a lesser extent than electroless metallization, which requires using highly harmful chemicals in several stages of the process, e.g., formaldehyde which is a common reducing agent. For economic reasons, there is a need to simplify the metallization process. Also for the sake of ecology and human health, efforts are made to eliminate harmful sensitizing agents. Therefore, it is worth considering why electroplating methods are not implemented on an industrial scale and if this situation can be changed. Introductory, all significant metallization methods of polymer materials were discussed. A distinction has been made between conductive polymers (intrinsic conductors) and conductive polymer composites (extrinsic conductors). Methods of modifying electrical conductivity are discussed for both intrinsic and extrinsic polymers. Finally, the results of the electroplating composites are reviewed and discussed.

The problem of metalizing polymers is well known and described in specialized literature. The conventional polymer material is an insulator and it is not possible to electroplate it. Technological development focused on the improvement of chemical and physical methods. These processes are complicated, require specialist, expensive equipment, and the chemicals used in them are often aggressive to the environment and humans. This chapter is intended to generally introduce the existing methods of creating metallic coatings on polymers and to familiarize with the basic problems that occur in this industry. The most commonly used in industry polymers that are metalized can be conventionally divided into three groups, depending on the maximum temperature range of their operation. This division is presented in Table 1.

The knowledge of the working temperatures of polymers is necessary to select the appropriate method of metallization. The most common in industry are electroless methods [4, 5], physical metal vapor deposition methods (PVD), and chemical vapor deposition (CVD) where either solid or liquid precursors are used [12‐14]. These methods can be briefly characterized as follows:

Electroless method (also known as chemical metallization). A metal deposition involves the flow of electrons from the more electronegative metal (or a reducing substance) to the metal ions of the deposited metal. This process takes place in the metallization bath without using an external source of power [15]. Electroless methods can be divided into three groups [15‐17]: deposition by exchange—metals less noble, more electronegative, replace the more electropositive, and noble from the solution. Deposition only takes place in direct contact between the metallization bath and the surface. After the entire surface is covered, the process stops; deposition by chemical reduction—a reduction from the ionic to metallic state occurs as a result of attaching an appropriate number of electrons to the metal ion. A substance that can donate electrons is called a reducer [18] and autocatalytic—is a kind of metal deposition by reduction. The process takes place in a bath in which metal ions are reduced in the presence of a catalyst. The reaction takes place when the catalyst is in direct contact with the bath and occurs only on the surface of the catalyst, which is the metal that is coated or deposited on the surface of the polymer product [19].

The autocatalytic deposition can be used in the metallization of polymers only if the surface of the material contains active areas resulting from the application of the catalyst [20]. The basic and most frequently used reducing agent in the autocatalytic metallization process is formaldehyde (CH₂O). This is a technological problem—the reducer is only effective at high pH levels (pH > 11), as well as an environmental problem because it is a highly poisonous agent, causes cancer, and is dangerous to the environment [21, 22]. Additionally, an aqueous palladium chloride solution is used during the sensitization step (PdCl₂). To reduce the number of wet chemicals steps, a one-step procedure has been developed with the use of a colloidal suspension containing Pd and Sn—an acidic solution of SnCl₂ and PdCl₂ [23‐25]. One of the most common metals deposited on the surface of polymers is copper. Due to the high electrical conductivity of Cu, autocatalytic copper plating plays an important role in the industry. The main area of application is in the electronics industry, production of printed circuits, integrated circuits as well as semiconducting structures [20, 26, 27]. The obtained copper layers can be further processed, by electroplating with other metals, such as nickel [23‐25].

PVD methods—The deposited metal is evaporated from the solid or liquid phase as particles. Then these particles in gaseous form are transported to and condensed on the surface of the polymer [24]. These methods make it possible to use pure metals as metallization reactants instead of compounds harmful to humans and the environment. The speed of the deposition process can reach 25 µm/s, while the thickness can be from one nano- to several millimeters [28]. Metallic layers obtained by these methods are characterized by high purity, good tribological properties, very high hardness, and high corrosion resistance [29]. However, the disadvantages of these methods are difficulties with the repeatability of the properties for obtained metallic layers, incomplete coverage of the metalized product in surface holes, as well as the price of the equipment, used [30]. A very important process parameter is the adhesion of the metal layer to the polymer substrate. It largely depends on the preparation and cleanliness of the surface of the product. The preparatory process consists of two main steps: chemical surface preparation (removal of all kinds of impurities, e.g., fats, greases) and/or activating the surface layer and heating it [31].

CVD methods—Metal particles are deposited on the surface of the product as a result of the chemical reduction or gas decomposition of a chemical precursor—a chemical compound that occurs or is formed in the first stage of the reaction, and which transforms into another substance in the further course of the phenomenon. Reactions can be initiated either thermally, by irradiation, or by plasma treatment [25, 32]. The reactions in CVD methods can be divided into decomposition, reduction, and chemical transport reactions [33, 34]. Each of them consists of many stages. These methods require using high temperatures, even as high as 1000 °C to initiate and maintain chemical reactions, hence the scope of application of this type of method, especially for polymer materials is limited [24, 31]. The rate of deposition of the metal layer depends on the temperature and gas pressure at which the process occurs. The latter parameter plays an important role in the rate of metal deposition—at low pressures, the transport of reactants from the gas phase is practically stopped. On the other hand, the growth rate of the deposited layer depends mainly on the product and the gas temperature. CVD methods require a lot of money and research on the properties of the precursors. The simple methods used to determine precursors' properties are thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) [35‐37]. The precursors must be of high chemical purity so that the resulting metal layers are free from inclusions of impurities. Therefore, the development of precursor purification methods such as sublimation, distillation, rectification, chromatography, sublimation is frequently applied [25].

Conventional polymers, due to the lack of electrical conductivity, are not suitable for electroplating. However, this technique is used as the next step in the entire coating process. The previously prepared polymer is subjected to the process of metal deposition by physical (PVD) or chemical (CVD or electroless) methods. Then, a metallic coating is created that enables a further electroplating process of the product.

Finding intrinsically conductive polymers opened the way to research the electroplating of these materials. This chapter briefly describes the types of intrinsically conductive polymers, followed by more significant research results.

Thermoplastic polymer composites are particularly important in high-volume production. They are a very good alternative to conventional construction materials such as steel, aluminum, or titanium. Relatively simple processing reduces the price of the product and the amount of waste. In addition to the low price, thermoplastic composites have the following advantages: low specific weight, capacity to damp vibrations, ease of product shaping, resistance to corrosion, and high fatigue strength. The disadvantages, however, are low-temperature resistance, strength, and stiffness as compared to the properties of conventional construction materials [74].

The hybrid composite called Schulatec TinCo 50 was tested by the Hansen team, the results of which are presented in [114]. The composite sample was the cathode, while the anode was pure copper. The electroplating bath was a solution of sulfuric acid and copper sulfate. The experiments were repeated for different values of the metallization time and current density. Increasing the current density from 3 to 8 A/dm² along with extending the processing time from 6 to 11 min resulted only in 12% coverage of the sample surface. The causes of such poor electroplating were also investigated. The sample was subjected to microscopic observation. Morphology analysis showed that only 7% of the composite surface had exposed copper fibers. On the other hand, the visible areas of Cu fibers were covered with a thin layer of polymer material. The level of adhesion was determined to be weak. The surface was mechanically treated by sandblasting and wet grinding. Due to these treatments, the surface of the sample had about 17% metallic coverage, which enabled electroplating of about 45% of the sample surface. The electrical conductivity of the samples subjected to electroplating was also tested. Even the incompletely covered sample (45% of the area) showed an increase in conductivity by about 70%. After the mechanical treatment, the deposited metallic layer was subjected to a tape test—it was not possible to remove the metal with the tape [114].

The paper [115] presents the research on the dependence on the effectiveness of electroplating on polymer 3D printed products. Conductive filaments available on the market, intended for 3D printing, such as Electrifi, Black Magic, and Proto-Pasta, were used for the tests. On acrylic substrates, samples for testing with sizes 0.1 × 0.5 × 8 cm were made. Copper electrodeposition was carried out in a two-electrode system, where the working electrode was a sample printed from a conductive filament, and the counter electrode was copper foil. The area of electroplating deposition was 4.25 cm². The electrolyte, an aqueous solution of CuSO₄ (1.0 M), H₂SO₄ (0.5 M), and NaCl (1 mM) was used for the metallization bath. A DC source was used for electrolysis and the metallization current density was 50 mA/cm². The calculated value of the copper deposition rate was 1.1 µm/min (with full current efficiency–50 mA/cm²). The research showed that the process of copper electrodeposition runs unevenly on the printed sample surface. A higher potential was near the electrical contact, which decreased along from this contact. This resulted in the deposition of less copper in the areas remote from the electrical contact. A sample made of commercial Proto-Paste was covered with a continuous layer of copper only 2 cm from the electrical contact, after one hour of electroplating. The high filament resistance caused the current density to drop below 3 mA/cm². Hence, the deposition of copper particles took place near the electrode contact.

The electroplating of the sample made of Black Magic showed after 5 min the formation of a metallic layer near the electrical contact. However, after one hour of the experiment, full coverage of the sample was obtained. The microscopic measurements showed inhomogeneity of the copper layer, which was also confirmed by the resistivity measurements. Surface resistivity increased with increasing distance from electrode contact. The resistance between 0 and 1.5 cm is nearly 7 times lower than that between 4.5 and 6 cm.

The electroplating of copper on a sample made of Electrifi resulted in a homogeneous metallic layer as initially proved by visual examination. On the other hand, surface images taken with an optical and electron microscope showed heterogeneity of the layer. Within 5 min, the deposition took place only around the electrical contact. After 15 min, larger areas of the sample were covered. In contrast, after 30 min of electroplating, the almost entire surface of the sample was covered with a metallic layer having locally present voids. Finally, after one hour of the process, the entire surface of the sample was covered with copper. The SEM images revealed that the copper deposition exhibited relatively high roughness and porosity. There have been no studies on the adhesion of the metallic layer to the substrate.

The work of Angel et al. presents the results of research on the electroplating deposition of details created thanks to 3D printing with the use of a two-head printer [116]. During the simultaneous operation of the 3D-printing process, one of the two heads fed non-conductive PLA filament, and the other conductive filament—Proto-paste. The samples were printed in two different directions: parallel to the work surface ("x–y" direction) and perpendicular to it ("z" direction). The conductivity of the product was tested with a multimeter in both directions: "x–y" direction – 81.9∙10^–3 S/cm and “z” direction – 43.3 10^–3 S/cm. An aqueous solution of copper sulfate was used for electroplating. The working electrode was an area printed from a conductive material, while the counter electrode was a copper foil placed in the bath parallel to the coated surface. The current source was a switching power supply, forcing a current flow of 0.04A which corresponded to a current density of about 2 mA/mm². The electroplating deposition process for the tested samples lasted 2, 4, 6, and 8 h. The deposition thickness of the metallic layer was determined using a confocal laser microscope. The metal layer deposition rates were obtained for the sample printed in the "x–y" direction at the rate of 24 µm/h and for the sample printed in the "z" direction—at the rate of 17 µm/h.

Images of the metallic layer morphology were taken using computed tomography (Zeiss Xradia 510 Versa CT scanner) for the samples subjected to electroplating for 8 h. Numerous cracks can be seen in the copper layer, especially in the sample printed in the "x–y" direction. The thickness of the deposited copper layer differed depending on the sample printing direction, and so for the "x–y" direction the layer had a thickness from 130 to 420 µm, and for the "z" direction this thickness was from 230 to 620 µm. The metallic layers on the conductive Proto-Paste filament, obtained in the process of electroplating had considerable roughness, different thicknesses, and discontinuities. The adhesion test according to the ASTM D3359 standard [105] was performed. The results showed that the adhesion of the obtained metallic layer to the substrate was rated 4A—“Trace peeling or removal along incisions or at their intersection” [116].

In the work of Dupenne et al. [113], the preparation of a polymer composite consisting of a polyurethane matrix (PU) and silver nanowires (AgNWs) was reported. AgNWs were synthesized by reducing AgNO₃ with ethylene glycol in the presence of poly(vinylpyrrolidone). The detailed course of the process was included in [117]. The conductivity of a single silver nanowire tested at room temperature was 6.3∙10⁵ S/cm [113]. Previously, the issue was dealt with by Sun et al. [118]. The use of AgNWs instead of spherical silver particles significantly lowered the percolation threshold (from 20% vol. to less than 5% vol.). The reason for this was the high aspect ratio of 222. With such a low filler content, it was possible to maintain the mechanical properties and low weight compared to spherical silver particles [113, 117]. The conductivity of undoped PU is 10^–12 S/cm. However, at 4% vol. filler content, the conductivity of the composite was 6.8∙10^–1 S/cm. The measurements were performed using the four-point method [113]. A percolation threshold of 0.6% vol. of the filler content was noted. Above the percolation threshold, effective electroplating of the composite was noted. The electrolysis process uses a silver electrode as the anode and a composite as the cathode. Electroplating was applied to PU composite samples with a content of 3% vol. to 6% vol. AgNWs. No effect of the degree of filler content on the metallization efficiency was observed. In this process, the duration of the electrolysis process and the current density were of decisive importance. The optimal conditions for electroplating were obtained with the share of 4% vol. filler and for a current density of 0.1 A/dm2. The deposition rate was recorded at the level of 7 µm/h [113]. The adhesion of the metallic layer to the polymer composite substrate was also determined using the ISO 2409 standard. The dependence of the adhesion on the current density of the electrolysis process was observed. For a density value lower than 0.2 A/dm2, the metal layer had the adhesion class 0—perfect adhesion. Increasing the value of the current density caused the deterioration of the adhesive properties of the metallic layer to the substrate.

The paper by Li et al. [119] presents the results of research on a composite consisting of a conductive polymer-polythiophene and nickel nanoparticles (NiNPs). PT was obtained by polymerizing thiophene monomers on FR4 substrate with KMnO₄. NiNPs particles were introduced into the polymer matrix using a proprietary method, not described in detail in the article. Two samples were prepared: one with NiNPs in the PT matrix and the other with undoped PT. Both samples were placed in a copper plating solution where the samples were the cathode and the copper electrodes were the anode. The experiment was carried out at a current density of 3.75 A/dm2. A detailed description of the process and rocks of the galvanic bath is given in [119]. After 30 min of the experiment, massive nuclei of copper particles appeared, while after an hour of electrolysis, complete coverage of the sample with a metallic layer was observed. The deposition rate of the copper layer on the PT + NiNPs composite was measured and it was 14.55 µm/h [119]. Such a rapid increase in copper layer deposition was initiated by the presence of NiNPs particles. The tests also showed that the sample without NiNPs particles was not covered with the copper layer. The thesis was formulated that NiNPs must be present in the PT matrix for the electroplating process to take place. Adhesion tests according to ISO 2409–1992 were also performed. The test results showed no damage to the metallic layer, which can be classified as class 0—excellent adhesion. There were also made PT + NiNPs tracks on the FR4 substrate, which were subjected to electroplating. As a result, conductive tracks with a resistance of 0.35 Ω were obtained, which powered the LED [119].

The conducted tests of conductive composites commercially available on the market in terms of the effectiveness of electroplating are presented in this section of the paper. It can be seen that in almost each of the cited examples there was a problem with the metallization of the polymer composite. The electroplating had possible for materials such as Schulatec TinCo 50, Proto-Pasta, or Black Magic. Unfortunately, it was not effective. The obtained metallic layer was not continuous and was present mainly in the vicinity of the electrical contact and the polymer electrode. Even mechanical processing—surface grinding (for TinCo 50) improved the metallization result up to 45% of the sample surface. The results of tests of filaments commonly used in 3D printers also gave a similar result. A higher electric potential could be observed near the electrode-polymer composite interface. Near this place, the deposition of the metallic layer was successful, but it disappeared with the increase of distance from the point of electrical contact. Similar problems occurred with the electroplating of conductive polymers (Sect. 3.2). The lack of uniform deposition of the metallic layer on the entire surface of the sample resulted in an extension of the experiment time. The obtained layers showed a different thickness and roughness. Only the use of silver nanowires as a filler significantly lowered the percolation threshold in the PU matrix. This enabled the electroplating process to be successfully carried out. The obtained metallic layer showed very good adhesion and smoothness. Similarly, doping the conductive polymer (PT) with nickel nanoparticles created a composite that can be efficiently and quickly electroplated.

Conventional polymers are typically insulators and their electroplating requires inducing changes in their composition and structure by chemical or physical methods. These indirect methods are part of the electroplating process and require multiple steps. Agents used to activate, oxidize, and etch the surface of polymers are highly polluting and pose a threat to human health. From the economic point of view, chemical and physical methods involve highly specialized equipment, which contributes to the final price of the product.

The main problem of implementing the method of electroplating of conductive composites is, above all, the ineffectiveness of the process. It is caused by the still too low electrical conductivity of polymers and polymer composite and the dependence of the electrical potential which decreases too high with increasing from the electrical contacting point. Research should be carried out on obtaining a higher electrical conductivity of the polymer composite as well as a uniform potential distribution on the surface of the metalized sample. The search should be aimed at finding the optimal filler content of various fractions on the scale of nanometers, micrometers, and millimeters, so that the composite has the mechanical properties of the polymer material. This is important due to the possible industrial processing of the product (e.g., 3D printing or injection molding). Only solving these problems will enable the implementation of the method on an industrial scale. Therefore, this technology needs further development and research with new total concepts which will be presented in our next successive publications.