Metallization of engineering plastics

Abstract

A material which has metallic like surface but in light weight is selected for different industrials, such as headlamp, crumple zone as well as hood in motor industry and rear mirror in optical application. Plastics are common materials for those applications because of their advantages, including light weight, low cost, ease of forming and corrosion resistance. Plastics are metallized as the presence of a functionalized surface (with carboxylate, -COO⁻ , group) and a certain degree of roughness. Different approaches, such as chemical and physical, have been studied to obtain these features. For industrial production, chemical approaches have been preferable used without limitation of size and shape. Chromium trioxide/sulphuric acid, H₂Cr₂O₇, is traditionally used as etching bath for acrylonitrile butadiene styrene (ABS). Due to the environmental concern and the increase demand of plastics other than ABS, other chemical approaches (e.g. alkali etching) instead of H₂Cr₂O₇ etching bath have been studied to modify a variety of engineering plastics, including polypropylene (PP), polycarbonate (PC) and 38% mineral filled poly(butylene) terephthalate-poly(ethylene) terephthalate (38% mineral filled PBT/PET) blend instead of ABS. Surface modification of the plastics was carried out by using varies solvents, such as sodium hydroxide, chloroform and polyethylene glycol. The surface modified plastics were activated by using Pd/Sn²⁺ colloidal activating bath and subsequent electroless Ni plating, and then followed by Cu-deposition through electroplating technique. By alkali etching, rough surfaces with carboxylate functional group were obtained. The correlation between the etching rate and the surface morphologies was investigated. Surface composition after etching, Ni coating and Cu coating were analyzed by Fourier transform infrared spectroscopy and X-ray diffraction. The roughness and the cross-section morphologies of the samples before as well as after etching, Ni coating and Cu coating were characterized by surface profile analysis, atomic force microscopy and scanning electron microscopy respectively. Using contact angle measurements, the effects of the etching time on the hydrophilic character of the plastics surface were assessed. Adhesion of the Cu layer, evaluated by the Scotch tape test, was found to be depended on the surface roughness and the palladium concentration of the activating bath. On the basis of our experimental results, we demonstrated that engineering plastics with ester functional groups can be etched by using alkali. As a result, carboxylate groups are procured to facilitate the formation of complex with Pd/Sn²⁺ colloidal in the activating bath. Besides a functional surface, a rough surface (10-20 μm) is also obtained by filling some insoluble salts into the plastics blends, so that the insoluble salts can react with alkali to produce pitting sites. It is important to control the surface roughness, because it directly affects the appearance of finished product and the adhesion between Cu coatings and the substrates. Generally, a plastic surface with optimal roughness and carboxylate groups promotes metallization.