A Facile Method Combined with Acetic Acid Modification and Electroless Plating to Fabricate Copper-plated Nylon 12 Powder for Antistatic Coating

Metal particle could deposited on Nylon 12 (PA12) surface using electroless plating with excellent interface and distribution, but the use of noble metal as catalytic site would increase the process cost and restrict its application. In this work, we employed a facile technology combined with acetic acid etching and electroless copper plating to prepare Cu/PA12 composite powder, and it used as conductive filler for antistatic coating was also studied. Results manifested defects (hole and amorphous structure) and amide group established on etched PA12 surface, which would facilitate the destruction of the [Cu-EDTA] structure, and then the reduction of REDOX barrier. As a result, Cu and Cu2O particles deposited on its surface. The downward trend of volume resistivity of antistatic coating appeared the rule of slow-fast-slow. The lowest volume resistivity was about 10 ohm·cm. This means that the dependable technology has great potential application in preparing metal/polymer composite material at a low cost.


Introduction
Electroless plating is free electrochemical deposition method, which is widely used in fabricating metal/polymer composite material. The development of electroless plating has drawn extensive attention mainly because it is a simple and promising method to prepare surface-metallization material [1,2]. Metal particles, such as Ag [3], Cu [4] and Ni [5], in-situ plated on substrate surface with excellent interfacial characterization and dispersion, which meet all the requirements on filler of antistatic coating [6]. The key of the electroless plating reaction is the establishment of catalytic site on polymer surface, so that the plating metal ions were catalyzed reduction metal coating process of formation [7,8].
It is obvious that without intermediate or cohesion in the interface between catalytic particle and substrate, the adhesion of plated coating would be greatly weakened, consequently, the modification of polymer surface is a essential technology [9]. As a result, the introduction of active group, the adsorption of catalyst and electroless plating (three-step process) are the common technology for electroless plating [10,11]. Huang et al. [9], Wang et al. [12] and Chang et al. [8] found that the modified polyethylene terephthalate surface with -NH2 and -SH on its surface could absorb Ag particles, acting as catalytic center, so that the Cu coatings are deposited through REDOX reaction in the presence of catalytic centers. By far the most catalytic sites, such as metal particle (Ni [13], Cu [14], Pd [5] and Ag [9]), defect [15] and active group [16] has been reported. All of these, like Pd and Ag, are noble metals, meaning that they are excellent catalytic capacity. But it has the characteristics of heavy pollution and high production cost. Non noble metal activation process has became increasingly important. Gui et al. [13] reported Ni particle/polymer brush was used as catalytic site on PA12 powders and Cu-plated PA12 powders were prepared. It is speculated that the absorption mechanisms of the polymer brush for metal ion include ion exchange, physical adsorption and charge neutralization. We also found that, the surface, at the vacancy edge, has more capacity to absorb incident energy [17]. This effect would break the structure of metal ion ligand and release trapped metal ion in the solution, which is consistent with catalytic mechanism of noble metal in plating solution. It must be mentioned that electroless copper plating using defect as catalytic site on Nylon 12 (PA12) surface has not been studied yet. Non metal activation and simple production make it easy and low-cost to practical use.
In this work, the defect-induced electroless copper plating (two-step plated process: modification and electroless plating) was reported instead of the conventional three-step plated process (modification, activation and electroless plating) on PA12 surface. Antistatic coating used the Cu-plated PA12 (CCP) powder as conductive filler was prepared. The mechanisms of defect-included electroless copper plating and conductive mechanism of antistatic coating were also investigated systematically.  Figure 1a shows the schematic diagram of electroless copper plating on PA12 powder. PA12 powder was immersed in acetic acid solution at 363 K for 30 min, then the CH3COOH-etched PA12 powder was obtained. After that, it immersed in electroless copper plating bath at 313 K for 60 min. The component of the solution is CuSO4 (15 g·L -1 ), C10H14N2Na2O8 (EDTA-2Na 10 g·L -1 ), KNaC4H4O6 ·4H2O (14 g · L -1 ), HCHO (15 mL·L -1 ), C10H8N2 (0.02 g·L -1 ) and NaOH (14 g·L -1 ). Then, CCP powder was obtained. Figure 1b shows the schematic diagram of the preparation of antistatic coating. The KH550 (20 g), deionized water (8 g) and ethanol (72 g) were added in a flask equipped, and mixed at a medium speed for 30 min, and then KH550 solution was obtained. CCP powder dropped into the system at 333 K, and mixed at a medium speed for 60 min. After that, the power cleaned by deionized water for 2 time, and then KH550-modified plated PA12 powders was obtained. After that, the KH550-modified powder and SiO2 particles (0.5%) were dispersed in styrene-acrylate emulsion. The mixture was left to stir for another 60 min, filtered the paint through 100 mesh sieve. The antistatic paint was then obtained. We put antistatic paint on glass, it was then dried to form film (10 mm×10 mm×0.5 mm), and then antistatic coating was obtained. The experiments' materials were purchased from Chengdu Kelong Chemical Reagent Company (China) and used without further purification.

Characterization
Surface morphology and chemical composition were studied by scanning electron microscopy (SEM, JEOL, JSM-5600LV) and energy dispersive spectrometer (EDS). The phase and orientation texture of the composite particles were determined by X-ray diffraction (XRD, Rigaku D/max-2550V). The chemical structure of the samples was measured by XRD (Rigaku D/max-2550V) and X-ray photoelectron spectroscopy (XPS, Shimazu, AXIS ULTRADLD). The binding energies obtained in XPS analysis were corrected with the reference to C 1s (284.6 eV). As displayed in Figure 2a, there are many holes on the PA12 surface after CH3COOH etching. We also analysed the surface area using Brunner-Emmet-Teller method, as displayed in Figure 2b and S1, the surface area of etched powder is 5.567 m 2 /g, it was 33 percent higher than the one of pristine powder (4.199 m 2 /g), which is consistent with the observation from SEM image. Taking the above observations into account, we conclude that the defect was established on PA12 surface after CH3COOH etching.

Results and discussions
The (200) and (220) plane possess the more attractive force between the crystal plane due to lower d. Reasonably, at first, the growth rate of (200) crystal plane is the fastest, the one of (111) is the slowest. In addition, the FCC plane free energy (G1) can be evaluated as following equation (2)  Based on the thermodynamical law, the particle preferably occupies the lowest energy state in a realistic crystal growth system. The FCC Cu and Cu2O promoted (111) growth and inhibited the other crystal orientation growth. As a result, preferred orientation of (111) crystal plane tends to be stronger gradually with the growth of grain. TG test was employed to evaluate Cu-deposited quantity on the CPP surface (Mplated particle/M(PA12+platedparticle)). Figure 3d represents TG spectrum of CPP powder. As shown in Figure 3d, the Mplated particle/M(PA12+platedparticle) is 18%, which is almost 72% of the one of CCP by Nicatalyzed electroless plating [13]. But it is more cost competitiveness compared with the reported work due to non metal activation and simple production.
Overall, a facile technology combined with etched modification and electroless copper plating to preparate Cu/PA12 composite powder was achieved. As shown in Figure 3e and f, it can be concluded that there are four important processes: First, defect form on PA12 surface after CH3COOH etching. Second, [EDTA-Cu] ligands is adsorbed around defect due to its high surface energy. Third, [Cu-EDTA] ligands stability is reduced and Cu 2+ is reduced to Cu atom by HCHO. At last, Cu atoms deposited on PA12 surface [13,20]. The key point is the intermediate step. The plating bath can be stable because of the formation of EDTA-Cu (ligand), and the REDOX barrier is enhanced, as a result, the Cu 2+ cannot be reduce by the reductant (HCHO). The reduction of [Cu-EDTA] stability would facilitate the electroless plating. The reduction process could be expressed as the following equation (4) and (5) [13,20].
Polar solvent molecule (CH3COOH) and amide group (-NHCO-) are tending to form excellent binding force (hydrogen bonding and polar bonds), and then break the binding force (intermolecalar force and hydrogen bonding) during polymer chains. The chains gradually dissociated form PA12 surface. This process is firstly breaking bond between the chain, then the reduction of its order, swelling at last. It can reduce the PA12 crystallinity from the outer to the center region. The surface energy and -NHCO-content would enhanced after etching.
In addition, as shown in Figure 3g, h and i, lone pair electrons form both N (2s2 2p3) and O (2s2 2p4) SP3 hybrid orbitals could occupied the Cu 2+ dsp2 hybrid orbital. As a result, a strong force can be establish between amide group and [Cu-EDTA], the force itself naturally takes on the form of coordinated bonds (Cu-N and Cu-O) [9,12]. The [Cu-EDTA] stability was reduced by the coordination effect, its structure was break and Cu 2+ was released [13], as displayed in Figure 3f, g and h. On the other hand, the swelling and dissolution would facilitate the formation of hole on the PA12 surface. The surface defect (amorphous structure and hole) and amide group of their high capillary imbibition and chemical activity also would reduce the [Cu-EDTA] stability [21], which would release trapped Cu 2+ . The free Cu 2+ was reduced by HCHO directly, as shown in Figure 3f. Antistatic paint (b), volume resistivity (c) and conductive mechanism (d) (e) of antistatic coating An antistatic paint (Figure 4b) was fabricated with styrene-acrylate resin as bonding agent and CCP powder as conductive filler. Figure 4c manifests volume resistivity of antistatic coating with different mass fraction of CCP powder. The test method is displayed in Figure S2. It was observed that the volume resistivity of antistatic coating with 10%, 20%, 30%, 35%, 40%, 45% and 50% CCP powder are 2.1×10 10 ohm·cm, 1.6×10 10 ohm·cm, 2.9×10 9 ohm·cm, 8.1×10 6 ohm·cm, 1.2×10 6 ohm·cm, 7.3×10 5 ohm·cm and 6.1×10 5 ohm·cm, respectively. Obviously, the downward trend of volume resistivity of antistatic coating appeared slow firstly and fast afterwards, at last S-curve. In addition, a considerable reduction of volume resistivity appears in the pattern (30-40% CCP), which is the so-called percolation threshold. The CCP powder is ''geometrically isolated'' at low filler content (Figure 4d) [22]. The conducting mechanism is the quantum tunnel effect of conductive particles resulting in the higher electronic transition barrier (∆G) and volume resistivity. The distance between neighboring plated powder would reduce with the increase of filler density, and then percolation channel was established when the content of filler up to a certain extent. (Figure 4e), the carrier can transport in the continuous plated PA12 network [23,24]. The ∆G is reduced significantly, therefore, volume resistivity is quickly reduced.

Conclusions
A facile technology combined with etched modification and electroless copper plating to fabricate CCP powder is successfully established. The hole, amide group and amorphous structure formed on CH3COOH-etched PA12 surface, which can break the structure of [Cu-EDTA] ligand, subsequently, the REDOX barrier of electroless copper plating was reduced. As a result, Cu and Cu2O spherical particles with a diameter of about 0.2-1 μm deposited on PA12 surface. The downward trend of volume resistivity of antistatic coating appeared the rule of slow-fast-slow. The lowest volume resistivity was approximately 10 5 ohm·cm.
This dependable technology has great potential application in preparing metal-polymer powder for additive manufacturing, conductive coating and electromagnetic shielding.