Preparation Of EMI Shielding Composite By Electroless Nickel-plated Wood With PdCl₂ Activation

Wood as a natural and renewable polymer material has many useful properties such as light in weight, heat and sound insulation, unique surface texture and color, and is widely used in the area of building decoration. However, dry wood has no conductivity of electricity, which limits its application in the field of electromagnetic shielding. Electroless nickel plating technique is an effective way to make wood metallized. Wood surfaces should be activated before plating. The traditional activation method employs colloidal Pd which is physically absorbed on wood surface. However, the Pd colloids tend to drop off and result in self-decomposition of the plating solution. Therefore, we prepared electroless nickel-plated wood veneers for electromagnetic interference(EMI) shielding by using 3-aminopropyl triethoxysilane(APTES) and chitosan based on the principle of N-Pd a coordination. The APTES and chitosan pretreatments in activation process were discussed respectively. The metal deposition, the surface resistance and the shielding effectiveness(SE) of the metallized wood veneers were measured. Also, the adhesion of the coating to the substrate was tested. The coating was characterized by SEM-EDS and XRD. Particularly, XPS was applied for surface analysis in activation process.The results of APTES-treated electroless nickel-plated wood veneers show that the optimum aging time of 1.5 v/v % APTES ethanol solution(containing 6 g/L water) is 5～7 h and the minimum surface resistivity of the plated birch and fraxinus mandshurica reaches 174.4 mΩ/cm2 and 210.4 mΩ/cm2, respectively. In the process of chitosan-treated electroless nickel-plated wood veneers, the optimum concentration of chitosan is 6 g/L and the optimum time of adsorbing PdCl2 is 8 min. The minimum surface resistivity of the plated birch and fraxinus mandshurica reaches 228.6 mΩ/cm2 and 236.9 mQ/cm2, respectively.XPS results show that the APTES film was successfully formed on wood surface and adsorbed Pd2+ ions through N-Pdσcoordination which were then reduced to Pd0 by NaH2PO2. SEM photographs indicate that the coating, covered completely on the whole wood surface, is uniform and continuous, and the texture of wood surface still remains. EDS and XRD results indicate that the coating is crystalline and is composed of Ni and P. The P content is lower than 4 wt.%.EMI shielding tests show that the SE of electroless nickel-plated wood veneers is significantly improved compared to the raw wood veneers. In frequencies from 9 kHz tol.5 GHz, the minimum SE of electroless nickel-plated birch and fraxinus mandshurica treated with APTES reaches 60 dB and 50 dB, respectively. Also, the minimum SE of electroless nickel-plated birch and fraxinus mandshurica treated with chitosan reaches 55 dB and 46 dB, respectively.The results of the vertical pulling tests indicate that the adhesion of the coating to the substrate is strong. The adhesion of the coating to the substrate treated with APTES is over 1.39 Mpa and those treated with chitosan is more than 0.98 MPa.