| Electroless plating on wood surface is an important method for wood functional modification, which can endow wood perfect electro-conductivity and electromagnetic shielding based on maintaining its own special properties. It can enlarge the application field for wood. In present, there exist some problems for electroless plating on wood, such as toxic reducing agent in copper plating on wood, high cost of activation process, and monotonous color of the coatings. In order to overcome those problems, novel electroless plating on wood were conducted for preparing electromagnetic shielding wood-based material.The birch and ash veneers were used as the substrate to investigate the suitable plating conditions according to surface resistivity and metal deposition. Four parts were carried out as follows:1) investigating the components of the plating solution and plating conditions for electroless copper plating by using glyoxylic acid as reductant under Pd2+activation;2) discussing the components of the plating solution and plating conditions for electroless copper plating under Pd-free activation that the activation and plating are combined in one bath;3) Endowing colorful surface to the coatings by coloring process for studying the conditions and the coloring mechanism;4) XPS, SEM-DES and XRD analysis were used to characterize the coating component, morphology and structure, respectively. The electro-conductivity, electromagnetic shielding performance, shade of the color, gloss, wettability, and adhesion strength were measured.The conclusions were summarized as below:(1) In the process of electroless copper plating by using glyoxylic acid as reductant, the suitable activation conditions are8g/L of chitosan treating for8min, Pd2+adsorption for8min and reduce in2g/L of sodium hypophosphite solution for8min. The plating solution composition and conditions for birch veneer are as follows:CuSO420g/L, EDTA·2Na40g/L, glyoxylic acid7.5g/L, Dipyridyl O.Olg/L, additive L0.015g/L, pH value12,55℃for30min. Those for ash veneer are as follows:CuSO412g/L, EDTA·2Na40g/L, glyoxylic acid10.5g/L, Dipyridyl O.Olg/L, additive L O.OlOg/L, pH value12,50℃for30min. The EDS and XPS results showed that the coating consists of native copper, which accords with the reducing mechanism of glyoxylic acid. The XRD analysis indicated that the coating is crystalline. SEM observation showed that the coatings with metallic luster covered the surface of wood uniformly. The electromagnetic shielding effectiveness (ESE) of plated birch and ash veneers are55dB and50dB in the frequencies from9kHz to1.5GHz, respectively. The adhesion strength is1.08Mpa and1.49Mpa. Surface decoration made the ESE decrease to44dB, which can still meet the practical application. The surface changed from hydrophilic to hydrophobic after electroless copper plating. In CIEL*a*b*color system, the change a*value of veneers before and after plated is significant. In Munsell system, the color code are9.8R6.9/10.6and9.8R7.1/10.1, indicating that orange-red with high lightness and saturation. The GZL and GZT parameters in gloss measurement of copper plated birch and ash veneers are7.75%,5.21%and6.15%,3.38%, respectively. They are higher than those of pristine wood veneers.(2) In the process of electroless copper plating by using Pd free activation, the pretreatment for birch veneer is that the veneers were immersed in3g/L of NaBH4solution for8min. The plating solution composition and conditions for birch veneer are as follows:CuSO420g/L, NiSO40.55g/L, glyoxylic acid8.5g/L, EDTA·2Na39g/L, Dipyridyl0.014g/L, additive L0.011g/L, pH value12.25,60℃for50min. Those for ash veneer are as follows:the pretreatment in3g/L of NaBH4solution for10min, CUSO418g/L, NiSO40.50g/L, glyoxylic acid8.5g/L, EDTA·2Na39g/L, Dipyridyl0.008g/L, additive L0.011g/L, pH value12.25,60℃for50min.The EDS, XPS and XRD results showed that the coating consists of native copper with the face-centre cubic structure, which no Ni and N-Cu alloy mixed. The ESE of plated birch and ash veneers are60dB and50dB in the frequencies from9kHz to1.5GHz, respectively. The adhesion strength is more than1.22Mpa and1.03Mpa. Surface decoration made the ESE decrease to49dB and37dB, which can still meet the practical application. The plated veneers have good hydrophobic property. The color codes are9.8R6.9/10.6and9.8R7.1/10.1for plated birch and ash veneers, which indicated full orange-red color. The GZL and GZT parameters in gloss measurement of copper plated birch and ash veneers are5.33%,4.35%and3.35%, 2.56%, respectively. They are lower than those of pristine wood veneers. The sheen is mild.(3) In the coloring process, the concentration of ammonium molybdate (0.25-0.35g/L) and sodium hypophosphite (0.8-1.Og/L) had no obvious effects on the surface resistivity of the colored veneer. The color of the veneer became darker and transited to red or blue. The temperature didn’t affect the surface resistivity of the colored veneer. However, temperature increase could promote the coloring reaction in some extents. In the middle of the color process, the color and the saturation of the veneer was the darkest and lowest. After25min of coloring process, the saturation gradually increased and the color was tending towards stable iridescence.The XPS and XRD results indicated that molybdenum oxide membrane formed and made the coating colorful after coloring process. Molybdenum oxide did not form crystalline structure. But nickel in the coating was still in crystalline style. It was observed by SEM photographs that the veneer surface was still covered by the coating with lower sheen. Coloring process made the ESE decrease, but it was still higher than40dB. Surface decoration made the ESE decrease to35dB, which can still reach the application standard. The adhesion strength is more than2.06Mpa. Coloring treatment made the coating more hydrophobic. The color changed from neutral color with moderate intensity to irregular iridescence before and after coloring process, which showed bad reproducibility. The sheen decreased and the anisotropic property faded down. Decoration made the colored veneer darker and the whole shade became pastel.The study of preparing wood-based electromagnetic shielding material and the properties by using novel electoless plating has resolved the key problems in the solution composition, activation conditions and coating colors. The obtained materials are environment friendly, colorful and with good properties, which offered the theory base and practical instruction for commercial produce and application. |
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