Study of Palladium Reduction and Its Effect on the Electroless Nickel Coating Adhesion on Carbon-Fibre Reinforced Polyetherimide Composite

Polymer composite is frequently coated with metals by various metallization processes, one of which is electroless nickel (EN) plating process. EN plating has been accepted by industry as a reliable method to produce the high quality, bright reflective surfaces. Aesthetically, the well-established bright metal finishes also offer plastic components with important functional advantages, such as wear resistance and hygienic properties. To improve EN coating adhesion on the polymer composite, chemical etching is used as the pre-treatment method prior to EN coating. However, the immediate consequence of the chemical etching is the exposure of the composite carbon fibre, leading to poor surface finish. To overcome the challenges, chemical-etching-free pre-treatment methods are desirable. In this paper, the key factors controlling the adhesion of EN coating on carbon fibre reinforced polyetherimide (CF-PEI) were investigated. Surface chemistry at different treatment stages was studied by X-ray photoelectron spectroscopy (XPS) analysis. It is found that reducing palladium (Pd) ions into Pd atoms on the composite surface takes a critical role to develop a good adhesion of EN coating to CF-PEI composite surface. When hypophosphite is used as a reducing agent, elevated temperature increases the density of catalytic Pd metal sites. With a stronger reducing agent sodium borohydride (NaBH₄), more adsorbed Pd ions can be reduced at room temperature than the high temperature hypophosphite. Even though NaBH₄ is used, there is still more than 30% adsorbed Pd ions yet to be reduced.